1.02.02 Adding and subtracting positive and negative numbers.

Adding negative numbers.

Imagine you have two bank accounts, one with a balance of £100 in it and a second account with a balance of -£20. If you combined these into a single account the balances from each would be added together and you would have a total balance of £80. (This is the same balance you would have had if you had just withdrawn £20 from your first account.) This illustrates rule 1:
Adding a negative number, is the same as subtracting a positive number of the same magnitude.


Subtracting negative numbers.

If your account has a balance of -£50 and your bank manager kindly agrees to "write off" your debt. i.e. to remove the balance of -£50, this would leave you with a balance of £0. (If you had deposited £50 into your account, this would have achieved the same result). This illustrates rule 2:
Subtracting a negative number, is the same as adding a positive number of the same magnitude.

In the above examples, once you have applied either of the rules to rearrange the sum, you are left with a simple familiar calculation. However this is not always the case and you may be faced with several possible unfamiliar procedures.
you may be required to:

The next page shows how to deal with these situations.