IGCSE · Practice questions · Using a calculator efficiently

Brackets, then round last.

Six original Cambridge-style questions on using a calculator efficiently: brackets, times, interpreting the display and keeping full accuracy.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

The method

Estimate, key in with brackets, round last.

Estimate first with 1 s.f. values, so you can spot a keying error.
Brackets round every numerator, denominator and expression under a root.
Times: minutes ÷ 60 gives the decimal of an hour; decimal × 60 gives minutes.
Write down the full display before rounding when asked for all the figures.
Money to 2 decimal places; other answers to 3 s.f.

Recall

[2 marks]

(a) Write 1.4 hours in hours and minutes. (b) Write 4 hours 36 minutes in hours, as a decimal.

(a) $0.4 \times 60 = 24$: 1 hour 24 minutes ✓
(b) $36 \div 60 = 0.6$: 4.6 hours ✓

Calculation

[2 marks]

Work out $\dfrac{8.3^{2} - 2.65}{1.7 + \sqrt{6.2}}$. Write down all the figures on your calculator display, then give your answer to 3 significant figures.

Numerator $= 68.89 - 2.65 = 66.24$; denominator $= 1.7 + 2.48997... = 4.18997...$ ✓
Display: $15.809\,144\,98...$ ✓
$15.8$ (3 s.f.) ✓

Calculation

[2 marks]

Work out $\sqrt{7.2^{2} + 3.5^{2}}$, giving your answer to 3 significant figures.

$7.2^{2} + 3.5^{2} = 51.84 + 12.25 = 64.09$ ✓
$\sqrt{64.09} = 8.0056... = 8.01$ (3 s.f.). Note: it is not $7.2 + 3.5 = 10.7$. ✓

Recall

[1 marks]

A calculation for the cost of some fabric gives the display $12.6$. Write down the cost in dollars.

$\$12.60$ ✓

Analysis

[3 marks]

A circle has area 60 cm². Ali finds the radius as 4.4 cm, then works out the circumference as $2 \times \pi \times 4.4 = 27.6$ cm.

Explain Ali's mistake and find the correct circumference to 3 significant figures.

Ali rounded the radius before using it. Keep the full value. ✓
$r = \sqrt{60 \div \pi} = 4.370\,19...$ cm ✓
Circumference $= 2\pi \times 4.370\,19... = 27.458... = 27.5$ cm (3 s.f.) ✓

Analysis

[4 marks]

A plumber charges a call-out fee of $\$45$ plus $\$38.50$ per hour. She starts a job at 09 50 and finishes at 12 30.

(a) How long does the job take?
(b) Calculate the total charge.

(a) 09 50 to 12 30 is 2 hours 40 minutes. ✓
(b) 2 h 40 min $= 2\dfrac{2}{3}$ h $= 2.666...$ h (not 2.4 h). ✓
Labour: $38.50 \times 2.666... = 102.666...$ ✓
Total: $45 + 102.666... = \$147.67$ ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.