Using a Reference Number 15
Using a reference number allows us to calculate problems such as
6 × 7, 6 × 6, 4 × 7 and 4 × 8.
Let’s see what happens when we try 6 × 7 using the method from
Chapter 1.
We draw the circles below the numbers and subtract the numbers
we are multiplying from 10. We write 4 and 3 in the circles. Our
problem looks like this:
6 × 7 =
–4
–3
Now we subtract crossways: 3 from 6 or 4 from 7 is 3. We write 3
after the equals sign.
6 × 7 = 3
–4
–3
Four times 3 is 12, so we write 12 after the 3 for an answer of 312.
6 × 7 = 3 12
–4
–3
Is this the correct answer? No, obviously it isn’t.
Let’s do the calculation again, this time using the reference
number.
10 6 × 7 = 30
–4
–3
- 12
42 Answer
Th at’s more like it.