58 Speed Math for Kids
To double 38, think of 40 – 2. Double 40 would be 80 and double
–2 is –4, so we get 80 – 4, which is 76. 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 you rself
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
Th is 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
- Both concepts are correct.