Subtraction follows naturally on from addition, because one way of viewing it is as the opposite of addition. For example, if you add 1 to 3, the answer is 4. If you subtract 1 from 4, the answer is 3, giving the original value.
Just as with addition there are trigger words in problems that tell you that you will need to subtract.
Consider the following questions:
- What’s the difference in distance between Milton Keynes and Edinburgh via the M6 or M1/A1?
- How much more money do you need to save?
- If you take away 45 of the plants for the front garden, how many will be left for the back garden?
- If all holiday prices have been decreased (or reduced) by £20, how much is this one?
All these questions involve the process of subtraction to find the answer – a process that you often meet when dealing with money. You can see that the ‘trigger’ words for subtraction are again in bold.
Remembering that subtraction is the opposite of addition gives one way of tackling problems involving subtraction using addition. A lot of people find addition lot easier than subtraction, so it’s a useful tip to remember.
For example, instead of saying, ‘£10 minus £7.85 leaves what?’ you could say, ‘What would I have to add to £7.85 to get to £10?’.
Adding on 5 pence gives £7.90, another 10 pence gives £8 and another £2 will give you a total of £10. So, the total amount to add on is £0.05 + £0.10 + £2.00 = £2.15. This is the same answer as the one obtained by subtraction: £10 − £7.85 = £2.15
This means that you can also check an answer to a subtraction problem by using addition.
You may need to carry out subtraction on paper though, so in the next section there is a quick reminder of how to do this.