c) 38 ÷ 2 =
d) 94 ÷ 2 =
e) 54 ÷ 2 =
f) 36 ÷ 2 =
g) 78 ÷ 2 =
h) 56 ÷ 2 =
The answers are:
a) 29
b) 38
c) 19
d) 47
e) 27
f) 18
g) 39
h) 28
To double 38, think of 40 − 2. Double 40 would be 80 and double −2 is −4, so we get 80 − 4, which is
- Again, this is useful when the units digit is high. With doubling, it doesn’t matter if the tens digit is
odd or even.
Test yourself
Now try these:
a) 18 × 2 =
b) 39 × 2 =
c) 49 × 2 =
d) 67 × 2 =
e) 77 × 2 =
f) 48 × 2 =
The answers are:
a) 36
b) 78
c) 98
d) 134
e) 154
f) 96
This strategy can easily be used to multiply or divide larger numbers by 3 or 4. For instance:
19 × 3 = (20 − 1) × 3 = 60 − 3 = 57
38 × 4 = (40 − 2) × 4 = 160 − 8 = 152
Note to parents and teachers
This strategy encourages the student to look at numbers differently. Traditionally, we have looked at a number like 38 as thirty, or
three tens, plus eight ones. Now we are teaching students to also see the number 38 as 40 minus 2. Both concepts are correct.
