Use a calculator to work out (7.52 − 4.2) ÷ (1.6 × 2.5), giving your answer to 3 significant figures.
- Top: 7.52 − 4.2 = 56.25 − 4.2 = 52.05
- Bottom: 1.6 × 2.5 = 4
- 52.05 ÷ 4 = 13.0125
- To 3 significant figures this is 13.0
Answer: 13.0
Using a calculator
Maths · Number. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
The idea
A calculator follows the order of operations. It does powers and roots first, then multiplying and dividing, then adding and subtracting. If you want a different order, you must put in brackets. This matters most when a fraction has a sum on the top or on the bottom.
You must also read the display with care. An answer in money needs two decimal places. A time such as 2.3 hours is not 2 hours 30 minutes. A display such as 4.5E−3 is a number in standard form.
Keep all the figures on the display while you work. Round only the final answer, to the accuracy the question asks for.
Rules to remember
- Order: brackets, then powers and roots, then × and ÷, then + and −
- For a fraction, put brackets round the whole top and round the whole bottom: (a + b) ÷ (c × d)
- A display of 3.2E5 means 3.2 × 105 = 320 000; 3.2E−5 means 3.2 × 10−5 = 0.000032
- Money: a display of 7.4 means $7.40
- Time: 2.3 hours = 2 hours and 0.3 × 60 minutes = 2 hours 18 minutes
- For sin, cos and tan of angles in degrees, the calculator must be in degree mode
- To square a negative number, use brackets: (−3)2 = 9
Common mistake
Students type a fraction such as (5 + 7) ÷ (2 × 3) without brackets, so the calculator works out 5 + 7 ÷ 2 × 3 instead. Put brackets round the top and the bottom every time.
Worked example
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