Multiplication Using Any Reference Number 47
23 × 31 = 713
5 4 11
2
Multiply our substitute numbers and then add the digits in the
answer:
5 × 4 = 20
2 + 0 = 2
Th is checks with our substitute answer, so we can accept that as
correct.
Test you rself
Here are some problems to try. When you have fi nished
them, you can check your answers yourself by casting out
the nines.
a) 21 × 24 = d) 23 × 27 =
b) 24 × 24 = e) 21 × 35 =
c) 23 × 23 = f) 26 × 24 =
You should be able to do all of those problems in your head. It’s not
diffi cult with a little practice.
MULTIPLYING NUMBERS BELOW 20
How about multiplying numbers below 20? If the numbers (or one
of the numbers to be multiplied) are in the high teens, we can use
20 as a reference number.
Let’s try an example:
18 × 17 =
brent
(Brent)
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