Multiplying numbers in the circles
The rule for multiplying the numbers in the circles is:
When both circles are above the numbers or both circles are below the numbers, we add the
answer. When one circle is above and one circle is below we subtract.
Mathematically, we would say: when we multiply two positive (plus) numbers we get a positive
(plus) answer. When we multiply two negative (minus) numbers we get a positive (plus) answer. When
we multiply a positive (plus) by a negative (minus) we get a minus answer.
Let’s try another problem. Would our method work for multiplying 8 × 42? Let’s try it.
We choose a reference number of 10. Eight is 2 below 10 and 42 is 32 above 10.
We either take 2 from 42 or add 32 to 8. Two from 42 is 40, times the reference number, 10, is 400.
Minus 2 times 32 is −64. To take 64 from 400 we take 100, which equals 300, then give back 36 for a
final answer of 336. (We will look at an easy way to subtract numbers from 100 in the chapter on
subtraction.)
Our completed problem looks like this:
We haven’t finished with multiplication yet, but we can take a rest here and practise what we have
already learnt. If some problems don’t seem to work out easily, don’t worry; we still have more to cover.
In the next chapter we will have a look at a simple method for checking answers.