3.1.1 Try some yourself
In a supermarket the bill comes to £8.70, and you have discount coupons worth £3.50. The assistant says ‘that will be £12.20 please’. Is she right?
No. You owe £8.70, but the coupons have paid £3.50, so you owe £8.70 − £3.50 = £5.20. The assistant wrongly added instead of subtracting.
Suppose a young friend, Tom, has run some errands for you, for which you have given him £20 plus some discount coupons for the supermarket. He presents you with your shopping, the following calculation and £1 change.
received from you £20.00
stamps from post office £5.00, leaves £15.00
supermarket bill £12.50, leaves £2.50
discount coupons £1.50, leaves £1.00
Has Tom done the right calculation?
No. Tom should have added not subtracted the value of the discount coupons, since they are the equivalent of money received, not spent. Alternatively he could have subtracted them from the supermarket bill before subtracting this: 15 − (12.50 − 1.50) is the same as 15 − 12.50 + 1.50.