Now let’s try the same example using 10 as a reference number:Add crossways, then multiply by 10 to get a subtotal:
18 + 7 = 25
10 × 25 = 250
Multiply the numbers in the circles and add this to the subtotal:
8 × 7 = 56
250 + 56 = 306
Our completed work should look like this:This confirms our first answer. There isn’t much to choose between using the different reference
numbers. It is a matter of personal preference. Simply choose the reference number you find easier to
work with.
Multiplying numbers above and below 20
The third possibility is if one number is above and the other below 20.
We can either add 18 to 14 or subtract 2 from 34, and then multiply the result by our reference
number:
34 − 2 = 32
32 × 20 = 640
We now multiply the numbers in the circles:
2 × 14 = 28
It is actually −2 times 14 so our answer is −28.
640 − 28 = 612
To subtract 28, we subtract 30 and add 2.
640 − 30 = 610
610 + 2 = 612