Chapter 9 — Two Full Practice Papers

Answer: B

Explanation

With $x$ loaves sold, the money from the loaves is $6x$ dollars, and the rent takes 4 of them away:

$$6x - 4 = 26 \quad\Longrightarrow\quad 6x = 30 \quad\Longrightarrow\quad x = 5$$

Check: $6(5) - 4 = 26$ ✓

A is the money from the loaves, $6x = 30\mathclose{,}$ not the number of loaves. C ignored the rent and divided 26 by 6. D subtracted the 4 where it has to be added back.

Takeaway: Add before dividing, and finish the last step.

Answer: D

Explanation

$$0.30 \times 240 = 72$$

C is the 70% who do not separate glass. B subtracted 30 and A added 30. A percentage of something is always a multiplication.

Takeaway: "$r$% of $b$" means $\frac{r}{100} \times b\mathclose{.}$

Answer: C

Explanation

Read the two marked points off the graph: after 1 minute the depth is 4 centimetres, and after 5 minutes it is 16 centimetres.

$$m = \frac{16 - 4}{5 - 1} = \frac{12}{4} = 3$$

B is upside down, A has the sign flipped, D added.

Takeaway: Rise over run, same order top and bottom.

Answer: D

Explanation

$$f(5) = 25 - 15 = 10$$

A added the 15 instead of subtracting it. C and B each computed one term and stopped.

Takeaway: Substitute into every term, then combine.

Answer: D

Explanation

$$x^2 + 6x - 4x - 24 = x^2 + 2x - 24$$

B has $-2x\mathclose{;}$ the $+6$ is bigger than the $-4\mathclose{,}$ so the middle is positive. C skipped the middle. A multiplied rather than added them.

Takeaway: Four products, then collect the middle two.

Answer: 32

Explanation

$$\frac{x}{4} = 8 \quad\Longrightarrow\quad x = 32$$

Check: $8 + 3 = 11$ ✓ Enter 32.

Takeaway: Subtract before multiplying up.

Answer: 23

Explanation

The four must total $4 \times 21 = 84\mathclose{.}$ The three given total 61, so the fourth is $84 - 61 = 23\mathclose{.}$ Enter 23.

Takeaway: Turn a mean into a total straight away.

Answer: C

Explanation

The two sentences say $y = 3x - 2$ and $2x + y = 13\mathclose{.}$ Substitute the first into the second:

$$2x + (3x - 2) = 13 \quad\Longrightarrow\quad 5x = 15 \quad\Longrightarrow\quad x = 3$$

Then $y = 7\mathclose{.}$ Check both: $y = 9 - 2 = 7$ ✓ and $6 + 7 = 13$ ✓ A reached $5x = 15$ and stopped. D lost the $-2$ altogether, reaching $5x = 13\mathclose{.}$ B gave $y\mathclose{,}$ the trays of cookies.

Takeaway: Substitute, solve, then check which variable was asked for.

Answer: B

Explanation

$(x^3)^4 = x^{12}\mathclose{,}$ then dividing subtracts: $x^{12-5} = x^7\mathclose{.}$

A added instead of multiplying at the first step. C skipped the division. D multiplied everything.

Takeaway: Power of a power multiplies; division subtracts.

Answer: C

Explanation

$5 + 3 = 8$ parts, so one part is $48 \div 8 = 6\mathclose{.}$ Dogs are 3 parts: $3 \times 6 = 18\mathclose{.}$

Check: cats $= 30\mathclose{,}$ and $18 + 30 = 48$ ✓ A is the ratio number itself, not a count of animals. D gave the cats. B divided the 48 by 3 instead of by the 8 parts.

Takeaway: Add the parts, find one part, then multiply.

Answer: D

Explanation

The graph crosses the $x$-axis at $x = -3$ and at $x = 5\mathclose{,}$ so $f(-3) = 0$ and $f(5) = 0\mathclose{.}$

A function that is zero at $-3$ has the factor $(x + 3)\mathclose{,}$ and one that is zero at 5 has the factor $(x - 5)\mathclose{:}$

$$f(x) = (x + 3)(x - 5)$$

Check one of them: at $x = 5\mathclose{,}$ $(5 + 3)(5 - 5) = 8 \times 0 = 0$ ✓

Why each wrong option is wrong:

• A, $f(x) = (x - 3)(x + 5)$ — is zero at 3 and at $-5\mathclose{:}$ both signs are the wrong way round.
• B, $f(x) = (x + 3)(x + 5)$ — is zero at $-3$ and at $-5\mathclose{.}$ The graph has a positive zero.
• C, $f(x) = (x - 3)(x - 5)$ — is zero at 3 and at 5. The graph has a negative zero.

Takeaway: A zero at $r$ means a factor of $(x - r)\mathclose{,}$ so the sign in the bracket is the opposite of the sign on the axis.

Answer: C

Explanation

The path is the hypotenuse of a right triangle whose legs are the two sides of the plot, 8 and 15:

$$64 + 225 = 289 \quad\Longrightarrow\quad c = 17$$

The 8–15–17 triple. D added the legs, B subtracted their squares, A averaged them.

Takeaway: Squares add, then take the root.

Answer: 11

Explanation

The distance between $x$ and 7 is $|x - 7|\mathclose{,}$ so $|x - 7| = 4\mathclose{.}$

$x - 7 = 4$ gives $x = 11\mathclose{;}$ $x - 7 = -4$ gives $x = 3\mathclose{.}$ The greatest is 11. Enter 11.

Takeaway: Two cases, then read the wording.

Answer: 80/3 or 26.66 or 26.67

Explanation

$45 \div 4 = 11.25$ bottles a minute, so 300 bottles take

$$\frac{300}{11.25} = \frac{80}{3} = 26.666\ldots \text{ minutes}$$

The decimal never ends, and the grid holds five characters, so it must be filled: enter 80/3, or 26.66 (cut off), or 26.67 (rounded). The shorter 26.7 is marked wrong.

Takeaway: Reduce to a per-one rate, then scale. A decimal that never ends has to fill the grid.

Answer: A

Explanation

$$3 = 4(2) + b = 8 + b \quad\Longrightarrow\quad b = -5$$

Check: at $x = 2\mathclose{,}$ $8 - 5 = 3$ ✓ The slope is $\frac{15 - 3}{5 - 2} = 4\mathclose{,}$ which is where the $4(2)$ comes from.

B is the $y$-value of the first point, not the value at $x = 0\mathclose{.}$ D added the 8 instead of subtracting it. C has the sign wrong: $3 - 8$ is negative.

Takeaway: Substitute the point into $y = mx + b$ and solve for $b\mathclose{.}$

Answer: D

Explanation

$$500 \times 1.2 = 600, \qquad 600 \times 1.2 = 720$$

A did one year only. C added 20% of the original twice, which ignores compounding — the second year's 20% is of 600, not 500. B worked out the second year's growth correctly, 120, and then added it to 500 instead of 600.

Takeaway: Percentage growth compounds: multiply, do not add.

Answer: D

Explanation

$$\frac{(x+4)(x-4)}{x+4} = x - 4$$

B kept the wrong factor. C and A cancelled terms, not factors.

Takeaway: Factor first, cancel whole brackets only.

Answer: D

Explanation

The pool is restricted to Spanish students, so the denominator is 70:

$$P = \frac{45}{70} = \frac{9}{14}$$

B used all 120. C is the chance of studying Spanish at all, $\frac{70}{120}\mathclose{.}$ A counted the other 25.

Takeaway: "Chosen from those who…" fixes the denominator.

Answer: B

Explanation

The number multiplied by $m$ is the rate: 30 cents per mile. A describes the 45. C would be $54. D invents something the model does not contain: a number of miles would not be multiplied by $m\mathclose{.}$

Takeaway: In $b + mx\mathclose{,}$ the multiplied number is the rate — check the units.

Answer: 9/2 or 4.5

Explanation

Let $x$ be the weight of a small box and $y$ the weight of a large box, in kilograms. Each sentence gives one equation:

$$4x + 3y = 26 \qquad\qquad 4x = y + 8$$

Write the second as $4x - y = 8\mathclose{.}$ Both equations now have $4x\mathclose{,}$ so subtract the second from the first:

$$3y - (-y) = 26 - 8 \quad\Longrightarrow\quad 4y = 18 \quad\Longrightarrow\quad y = \frac{9}{2}$$

A large box weighs 4.5 kilograms.

Check: the second equation gives $4x = 8 + 4.5 = 12.5\mathclose{,}$ and then $12.5 + 3(4.5) = 12.5 + 13.5 = 26$ ✓

Enter 9/2 or 4.5.

Takeaway: One sentence, one equation. Matching terms, subtract — and bracket the second equation so the double negative is visible.

Answer: 25

Explanation

One solution means $b^2 - 4ac = 0\mathclose{:}$

$$100 - 4c = 0 \quad\Longrightarrow\quad c = 25$$

Then $x^2 + 10x + 25 = (x+5)^2$ ✓ Enter 25.

Takeaway: Exactly one solution ⇒ discriminant zero ⇒ perfect square.

Answer: A

Explanation

The right side is $r^2 = 36\mathclose{,}$ so $r = 6\mathclose{.}$ B is $r^2\mathclose{,}$ not $r\mathclose{.}$ C halved the 36 instead of taking its square root. D is the centre, $(-2, 5)$ — a point, not a length, and the answer to a different question.

Takeaway: Take the square root of the right-hand side.

Answer: B

Explanation

Areas scale by the square of the length ratio, so take square roots:

$$\sqrt{\frac{16}{49}} = \frac{4}{7}$$

So the lengths are in the ratio $4 : 7\mathclose{.}$ A left the area ratio unchanged. C squared instead of rooting. D took the root and then wrote it upside down.

Takeaway: Lengths $k\mathclose{,}$ areas $k^2\mathclose{,}$ volumes $k^3\mathclose{.}$ Go backwards with a root.

Answer: B

Explanation

Inside first: $f(4) = 7\mathclose{.}$ Then $g(7) = 14\mathclose{.}$

A is $f(g(4)) = 11\mathclose{,}$ the other order — always offered. C added the two separate outputs, $f(4) + g(4) = 7 + 8\mathclose{.}$ D multiplied the three numbers in sight.

Takeaway: $g(f(x))$ means $f$ first. Work outward from the inner bracket.

Answer: A

Explanation

$$35n + 75 \le 500 \quad\Longrightarrow\quad 35n \le 425 \quad\Longrightarrow\quad n \le 12.1$$

You cannot load part of a sack, so round down to 12.

Check: $12 \times 35 + 75 = 495 \le 500$ ✓ and $13 \times 35 + 75 = 530 > 500$ ✓

C forgot the generator. B rounded up — a count of physical objects always rounds down against a maximum. D did both.

Takeaway: Include everything that adds to the load, then round down.

Answer: C

Explanation

The side next to the angle $\theta$ is 3 and the hypotenuse is 5. By Pythagoras the third side, the one opposite $\theta\mathclose{,}$ is $\sqrt{25 - 9} = 4\mathclose{:}$

$$\sin\theta = \frac{\text{opposite}}{\text{hypotenuse}} = \frac{4}{5}$$

A is the cosine, adjacent over hypotenuse. D is greater than 1, which no sine can be. B is adjacent over opposite, $\frac34$ — neither a sine nor a cosine.

Takeaway: Find the missing side first, then read off the ratio you need: sine is opposite over hypotenuse.

Answer: 48

Explanation

The new price is 125% of the old:

$$1.25 \times \text{old} = 60 \quad\Longrightarrow\quad \text{old} = \frac{60}{1.25} = 48$$

Check: 25% of 48 is 12, and $48 + 12 = 60$ ✓

Taking 25% off $60 gives $45, not $48 — which is exactly why working backwards needs division, not a second percentage. Enter 48.

Takeaway: To undo a percentage change, divide by the multiplier.

Paper 1 — Module 2

27 questions · 43 minutes · calculator permitted

Take a short break before starting. On the real test the second module's difficulty depends on how you did in the first; here it simply continues.

Answer: C

Explanation

$$-2x = -8 \quad\Longrightarrow\quad x = 4$$

Check: $9 - 8 = 1$ ✓ B kept the minus after dividing two negatives. D added the 1 to the 9 instead of subtracting it, reaching $2x = 10\mathclose{.}$ A made both slips.

Takeaway: A negative divided by a negative is positive.

Answer: C

Explanation

$$f(0) = 4(0) + 7 = 7$$

A assumed an input of 0 gives an output of 0. B gave the slope. D added the two numbers in the formula, which is $f(1)\mathclose{.}$ The 7 is the handling fee: the charge before any distance is driven.

Takeaway: $f(0)$ is the $y$-intercept — the constant term.

Answer: D

Explanation

$$A = 12 \times 7 = 84$$

A added the two sides. B is the perimeter. C halved, which belongs to a triangle.

Takeaway: Area multiplies; perimeter adds. Rectangles have no halving step.

Answer: A

Explanation

$$3(2x - 5) = 6x - 15, \qquad 6x - 15 + 4x = 10x - 15$$

B left the $-5$ undistributed. C dropped the $x$ from $4x\mathclose{.}$ D made both of those slips at once.

Takeaway: Distribute across every term, then collect like with like.

Answer: D

Explanation

Kilometres over centimetres is the same for the map's scale and for the two picnic sites. With $x$ kilometres between them:

$$\frac{x}{6} = \frac{10}{4}$$

Cross-multiply: $4x = 60\mathclose{,}$ so $x = 15\mathclose{.}$

Check: $\frac{15}{6} = 2.5$ and $\frac{10}{4} = 2.5$ ✓

A cross-multiplied the wrong pair, $10x = 24\mathclose{.}$ B is the number of kilometres that one centimetre stands for, not the distance. C multiplied 4 by 10 and divided by 6.

Takeaway: Cross-multiply, then check both fractions come to the same decimal.

Answer: 5

Explanation

On the ground the height is zero:

$$-5t^2 + 20t + 25 = 0$$

Divide every term by $-5$ to make it easy to factor:

$$t^2 - 4t - 5 = 0 \quad\Longrightarrow\quad (t - 5)(t + 1) = 0 \quad\Longrightarrow\quad t = 5 \text{ or } t = -1$$

A time of $-1$ is before the ball was thrown, so it is rejected. The ball lands after 5 seconds. Check: $-5(25) + 20(5) + 25 = -125 + 100 + 25 = 0$ ✓ Enter 5.

Takeaway: "Hits the ground" means height zero. Solve both roots, then let the situation choose: a time cannot be negative.

Answer: 15

Explanation

The fall is $12. Percentage change is measured against the original:

$$\frac{12}{80} \times 100 = 15$$

Using 68 as the base would give 17.6%, which is the standard wrong answer. Enter 15.

Takeaway: Percentage change divides by the original value, always.

Answer: A

Explanation

Parallel lines have equal slopes, so the slope is $-3\mathclose{.}$ C dropped the sign.

B flipped and negated, which gives a perpendicular slope, $\frac13\mathclose{.}$ D read the intercept.

Takeaway: Parallel copies the slope exactly, sign included.

Answer: D

Explanation

$$V = 5 \times 4 \times 3 = 60$$

A added the three edges. B multiplied only two of the three edges: that is the area of one face. C is half the surface area. Check what the question asked for — a volume is in cubic units, an area in square ones.

Takeaway: Volume multiplies all three dimensions.

Answer: C

Explanation

Read the six heights off the graph: 4, 9, 2, 11, 6 and 8.

Sorted: $2, 4, 6, 8, 9, 11\mathclose{.}$ Six values, so average the middle two:

$$\frac{6 + 8}{2} = 7$$

B is the mean. A and D picked one middle value without averaging.

Takeaway: Sort first. Even count means average the middle pair.

Answer: B

Explanation

$$5^{-2} = \frac{1}{5^2} = \frac{1}{25}$$

A negative exponent gives a small positive number, never a negative one.

A squared the 5 and made it negative. D ignored the minus sign on the exponent. C multiplied 5 by 2.

Takeaway: Negative exponent means reciprocal, not negative.

Answer: D

Explanation

No solution means parallel: equal slopes, different intercepts. So $k = 4\mathclose{,}$ and the intercepts 1 and $-3$ differ ✓

A has the sign of the slope wrong. C matched intercepts, which would make the lines cross, not miss. B copied the other line's intercept.

Takeaway: Parallel and distinct. Check the intercept after matching the slope.

Answer: 4

Explanation

Let an adult ticket cost $x$ dollars and a child ticket $y$ dollars. The two sentences say

$$5x + 2y = 26 \qquad 3x - 2y = 6$$

The $y$ terms are $+2y$ and $-2y\mathclose{,}$ so adding the equations removes them:

$$8x = 32 \quad\Longrightarrow\quad x = 4$$

Then $5(4) + 2y = 26$ gives $y = 3\mathclose{.}$ Check both: $20 + 6 = 26$ ✓ and $12 - 6 = 6$ ✓ Enter 4.

Takeaway: Opposite signs on a variable, add the equations. Then check both originals.

Answer: 7/4 or 1.75

Explanation

$$x = -\frac{5}{2}, \qquad f\left(-\tfrac{5}{2}\right) = \frac{25}{4} - \frac{25}{2} + 8 = \frac{7}{4}$$

Check: $f(-3) = 9 - 15 + 8 = 2$ and $f(-2) = 4 - 10 + 8 = 2$ — higher on both sides ✓

The question asked for the minimum value, which is the $y\mathclose{,}$ not the $-\frac{5}{2}\mathclose{.}$ Enter 7/4 or 1.75.

Takeaway: Find $x = -\frac{b}{2a}\mathclose{,}$ substitute, and check whether the question wants where or what.

Answer: A

Explanation

The constant is the value at $x = 0\mathclose{.}$ B describes the 6. C and D are things the model says nothing about: a line of best fit describes a relationship, not the size of the sample or the range of ages.

Takeaway: Constant = value at zero; coefficient = rate. Check the units.

Answer: C

Explanation

Each hour the mass is multiplied by $0.85\mathclose{:}$ "is 85% of" gives the factor directly, $b = 0.85\mathclose{.}$

$$M = 400(0.85)^h$$

A grows by 15% an hour instead of shrinking. B is linear: it removes 0.85 grams an hour. D used $0.15\mathclose{,}$ the fraction lost.

Takeaway: "Decays to 85%" means $b = 0.85\mathclose{;}$ "decays by 15%" means the same thing. Read which word is used.

Answer: D

Explanation

$$m = \frac{16 - 4}{3 - (-1)} = \frac{12}{4} = 3$$

So $y = 3x + b\mathclose{.}$ Substitute $(-1, 4)\mathclose{:}$ $4 = -3 + b\mathclose{,}$ giving $b = 7\mathclose{.}$

Check with the other point: $3(3) + 7 = 16$ ✓

C read a coordinate off a given point; B gave the slope. A put $x = 1$ in for the point $(-1, 4)\mathclose{,}$ losing its sign: $4 = 3 + b\mathclose{.}$

Takeaway: Slope first, then substitute either point for $b$ — and confirm with the point you did not use.

Answer: B

Explanation

Square both sides:

$$3x + 1 = 16 \quad\Longrightarrow\quad 3x = 15 \quad\Longrightarrow\quad x = 5$$

Check in the original: $\sqrt{3(5) + 1} = \sqrt{16} = 4$ ✓ — and with radicals the check is compulsory, because squaring can invent solutions.

A added 1 to both sides rather than squaring. C forgot to subtract the 1, and D added it instead of subtracting.

Takeaway: Square, solve, then substitute back into the original.

Answer: A

Explanation

$\frac{40}{360} = \frac19$ of the circle. The whole circumference is $2\pi(9) = 18\pi\mathclose{,}$ so the arc is

$$\frac19 \times 18\pi = 2\pi$$

C is the sector area, a different quantity — an arc is a length. B used 18 as the radius, which is the diameter. D is the whole circumference, with the fraction never applied.

Takeaway: Fraction of the circle, times the circumference for an arc, times the area for a sector.

Answer: 21

Explanation

Adding the two statements gives $2x = 66\mathclose{,}$ so the larger is 33 and the smaller is $54 - 33 = 21\mathclose{.}$

Check: $21 + 33 = 54$ ✓ and $33 - 21 = 12$ ✓ Enter 21.

Takeaway: The smaller of two numbers is $\frac{\text{sum} - \text{difference}}{2}\mathclose{.}$

Answer: 81

Explanation

A perfect square is $(x - a)^2 = x^2 - 2ax + a^2\mathclose{.}$ Here $2a = 18\mathclose{,}$ so $a = 9$ and

$$c = a^2 = 81$$

Check: $(x - 9)^2 = x^2 - 18x + 81$ ✓ Enter 81.

Takeaway: Halve the middle coefficient, then square it.

Answer: D

Explanation

$$800 \times 1.25 = 1000, \qquad 1000 \times 1.25 = 1250$$

B did one year only. C used simple interest — 25% of the original $800 twice, which is $400 rather than $450. Compounding earns more, because the second year's interest is on $1000, not $800. A added $25 a year instead of 25%.

Takeaway: Compound interest multiplies by the factor once per period. Compounding always beats simple interest over more than one period, and the gap is the whole point of the question.

Answer: A

Explanation

The $-5$ is outside the bracket, so it moves the graph down 5: $(3, 8) \to (3, 3)\mathclose{.}$

B moved sideways, which only happens inside the bracket. C swapped the coordinates. D moved the graph up 5 instead of down.

Takeaway: Outside the bracket moves vertically, in the direction the sign says.

Answer: C

Explanation

The bars show 5 red, 7 blue and 8 green counters. That is 20 counters, of which $5 + 7 = 12$ are not green:

$$\frac{12}{20} = \frac{3}{5}$$

Or take the complement: $1 - \frac{8}{20} = \frac{12}{20}$ ✓

B answered the opposite question. D compared the green counters with the other twelve instead of with all twenty. A assumed three colours means three equally likely outcomes; the counts are not equal.

Takeaway: "Not" means subtract from 1, or count the ones you want directly. Both should agree.

Answer: B

Explanation

$$2w = P - 2l \quad\Longrightarrow\quad w = \frac{P - 2l}{2}$$

A skipped the division. C divided only the $P\mathclose{.}$ D moved the $2l$ across without changing its sign.

Takeaway: Whatever you divide by must divide every term.

Answer: A

Explanation

Flip and negate: $-\frac74\mathclose{.}$ Check: $\frac47 \times -\frac74 = -1$ ✓

C is the slope of a parallel line. D and B each did one half of the job.

Takeaway: Negative reciprocal is two steps. Multiply the two slopes; you should get exactly $-1\mathclose{.}$

Answer: 6

Explanation

Let the width be $w\mathclose{.}$ Then the length is $2w + 3\mathclose{,}$ and the perimeter is

$$2(2w + 3) + 2w = 42$$ $$4w + 6 + 2w = 42 \quad\Longrightarrow\quad 6w = 36 \quad\Longrightarrow\quad w = 6$$

Check: width 6, length 15, perimeter $2(15 + 6) = 42$ ✓

The question asked for the width, not the length — and with no options to warn you, re-reading the final line is the whole safety net. Enter 6.

Takeaway: Define the letter, build the equation, solve, then check which of the two quantities was asked for. This is the last question of the module; do not rush it.

Paper 2 — Module 1

27 questions · 43 minutes · calculator permitted

Sit this one at least a week after Paper 1, and mark Paper 1 first — a mock is only useful if you have acted on what the last one told you.

Answer: A

Explanation

With $x$ parcels the charge is $5 each plus the $12 fee, so $5x + 12 = 47\mathclose{:}$

$$5x = 35 \quad\Longrightarrow\quad x = 7$$

Check: $35 + 12 = 47$ ✓ C added the 12 instead of subtracting it. D stopped one step early. B ignored the fee and divided the whole 47 by 5.

Takeaway: Subtract, then divide, then finish.

Answer: A

Explanation

$$-2(x - 5) = -2x + 10, \qquad 7x - 2x + 10 = 5x + 10$$

B kept the constant negative — but $-2 \times -5 = +10\mathclose{.}$ C turned the subtraction of $2x$ into an addition. D subtracted the $2x$ and then carried the 5 down without multiplying it by anything.

Takeaway: A minus before a bracket flips every sign inside it.

Answer: A

Explanation

The line meets the $y$-axis at the point $(0, -6)\mathclose{,}$ so the $y$-coordinate is $-6\mathclose{.}$

C is the slope: the line rises 3 for every 2 it moves to the right. D is where the line crosses the $x$-axis, at $(4, 0)\mathclose{.}$ B is the value of $x$ on the $y$-axis, not of $y\mathclose{.}$

Takeaway: On the $y$-axis, $x = 0\mathclose{.}$ Each axis zeroes the other variable, so read the $y$-intercept where the line meets the vertical axis.

Answer: C

Explanation

With $x$ T-shirts the order costs $7x$ dollars before the voucher and $7x - 9$ after it:

$$7x - 9 = 33 \quad\Longrightarrow\quad 7x = 42 \quad\Longrightarrow\quad x = 6$$

Check: $42 - 9 = 33$ ✓

A subtracted the 9 where it has to be added back; D is the cost before the voucher, $7x = 42\mathclose{,}$ not the number of T-shirts; B ignored the voucher.

Takeaway: Undo the subtraction first, then the multiplication — and finish.