Imagine that you are buying a t-shirt for £9.99, a pair of socks for £1.49 and a belt for £8.99 The cashier charges you £23.47. You feel that this is too much - but how do you know?
One way of finding out whether you have been over-charged is to estimate what the total amount should be. Round the different prices into easier numbers - £9.99 is approximately £10, £1.49 is approximately £1.50 and £8.99 is approximately £9 - and you can do the calculation quickly in your head.
£9.99 + £1.49 + £8.99 ≈ £10 + £1.50 + £9 = £20.50
This is almost £3 less than the cashier asked for, so obviously you have been over-charged.
Note: the symbol ≈ means 'approximately equal to'.
By rounding the actual values to more manageable numbers, you can estimate the answers to many problems:
£2.99 + £3.10 + 99p ≈ £3 + £3 + £1 = £7
29 × 9 ≈ 30 × 10 = 300
61 ÷ 6 ≈ 60 ÷ 6 = 10
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