Multiplication using two reference numbers

The general rule for using a reference number for multiplication is that you choose a reference number that is close to both numbers being multiplied. If possible, you try to keep both numbers either above or below the reference number so you end up with an addition.

What do you do if the numbers aren't close together?

What do you do if it is impossible to choose a reference number that is anywhere close to both numbers?

Here is an example of how our method works using two reference numbers:
8 × 37 =

First, we choose two reference numbers. Th e first reference number should be an easy number to use as a multiplier, such as 10 or 100. In this case we choose 10 as our reference number for 8.

The second reference number should be a multiple of the first reference number. That is, it should be double the first reference number, or three times, four times or even sixteen times the first reference number.

In this case I would choose 40 as the second reference number, as it equals 4 times 10. We then write the reference numbers in parentheses to the side of the problem, with the easy multiplier written first and the second number written as a multiple of the first.

So, we would write 10 and 40 as (10 × 4), the 10 being the main reference number and the 4 being a multiple of the main reference.