We check by casting out the nines:Multiply our substitute numbers and then add the digits in the answer:
5 × 4 = 20
2 + 0 = 2
This checks with our substitute answer so we can accept that as correct.Test yourself
Here are some problems to try. When you have finished them, you can check your answers yourself by casting out the nines.
a) 21 × 24 =
b) 24 × 24 =
c) 23 × 23 =
d) 23 × 27 =
e) 21 × 35 =
f) 26 × 24 =
You should be able to do all of those problems in your head. It’s not difficult 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 =
Using 20 as a reference number we get:
Subtract diagonally:
17 − 2 = 15
Multiply by 20:
2 × 15 = 30
30 × 10 = 300
Three hundred is our subtotal.
Now we multiply the numbers in the circles and then add the result to our subtotal:
2 × 3 = 6
300 + 6 = 306
Our completed work should look like this: