Numbers, units and arithmetic
Numbers, units and arithmetic

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Numbers, units and arithmetic

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Activity 23

Candles are sold in boxes of 40. How many candles are there in 30 boxes?


One box has 40 candles.

10 boxes have 40 × 10 candles = 400 candles.

30 boxes have 40 × 3 × 10 candles or 40 × 10 × 3 or 400 × 3 or 1200 candles.

Activity 24

  • 2(a) A doctor prescribes 3 pills a day and the prescription is to last for 4 weeks. How many pills should the pharmacist supply?

  • (b) Apples are sold in a pack of 7 for 95 p or singly 14 p each. Which is the better buy?


  • (a) There are 7 days in a week, so 4 weeks is 7 × 4 = 28 days.

  • The number of pills required is 28 × 3 = 84.

  • (b) The apples in packs cost p each.

  • Dividing 95 by 7 gives 13 ,

  • which is less than the 14p per apple sold singly.

  • Alternatively, you could compare the cost of 7 apples: 7 apples at 14p each would cost 98p, which is more than 95p. So the apples in the pack would appear to be the better buy. (However, if you do not want 7 apples, and are likely to leave some of them to rot, you would be better off just buying the number of apples you want!)

Activity 25

Describe, in your own words, the effect of multiplying any number by (a) 1, (b) 0.


  • (a) Multiplying by 1 gives an answer the same as the original number. (There are many possible ways of wording this. You may have said it ‘leaves the number unchanged’ or ‘has no effect’.)

  • (b) Multiplying by 0 always gives 0.

Activity 26

Place the following times in order, shortest first:

  four weeks;  29 days;  360 hours.


Convert all the times to days, say:

  four weeks is 4 × 7 = 28 days;

  29 days is already in days.

To convert 360 hours to days, divide by 24.

Writing it as you may spot that you can divide top and bottom by 12 to give = 15.

(Or you can divide 360 by 12 and then by 2 to give the same result as dividing by 24.

Alternatively you may say 10 days is 240 hours, 5 days is 120 hours, so 15 days is 360 hours.)

So, in order:

  360 hours, 4 weeks, 29 days.


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