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PSLE 2015 · Paper 2 · Q13 Measurement & Geometry · Area & perimeter 4 marks: 1 + 3 difficulty 4 of 5

PSLE 2015 Paper 2, Question 13: Area & perimeter

The answer

(a) \(120 \text{ cm}^2\) (b) \(48\)

PSLE 2015 Paper 2 Question 13 · Verified worked solution by the Genius Plus Academy teaching team

What this question tests

This is Question 13 of the PSLE 2015 Paper 2 Mathematics paper. It tests area & perimeter, in the Measurement & Geometry strand. It is worth 4 marks: 1 + 3. Open your copy of the paper at this question, then follow our full worked solution below.

Watch the walkthrough

Video: a Genius Plus Academy teacher solving PSLE 2015 Paper 2 Question 13

Teacher video · 2015 P2 Q13

Step-by-step solution

GPA solution: cutting layout of 24×10 rectangles in the 60×100 cardboard — GPA solution: the 100 cm × 60 cm cardboard with 10 × 24 rectangles arranged, 10 columns × 2 rows upright, plus 4 sideways rectangles in the remaining 12 cm strip; each rectangle gives 2 triangles.

(a) The legs are 10 cm and 24 cm (the right angle is between them): area \(= \frac{1}{2} \times 10 \times 24 = 120 \text{ cm}^2\).

(b) Two triangles pair into a \(10 \times 24\) rectangle. In the \(60 \times 100\) board: \(100 \div 10 = 10\) columns and \(60 \div 24 = 2\) rows (remainder 12 cm) → 20 rectangles; in the leftover \(12 \times 100\) strip lay rectangles sideways: \(100 \div 24 = 4\) → 4 more. Total \(24\) rectangles \(= 48\) triangles.

Answer: (a) \(120 \text{ cm}^2\) (b) \(48\)

Same structure, different costume

A question is hard because of its structure, not its story.

Change egg tarts to badges, a tank to two tanks, a ring of card to a folded sheet, and a child sees a brand new problem. The lock underneath is the same, and so is the key. Once a student can name the structure, a whole row of questions that look different start to open the same way.

It is also where marks really leak. Across fourteen years of papers, the marks lost in PSLE math leak at the reading far more than at the arithmetic, which is exactly why recognising the structure fixes them. We call this Lock and Key: name the lock, then the key follows.

See what the paper rewards → The hardest questions, shown working →

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