ffirs.indd

64 Speed Math for Kids
The answers are:
a) 15 b) 25 c) 45 d) 35
Th is is an easy way to learn the 5 times multiplication table.
Th is works for numbers of any size multiplied by 5. Let’s try 14 × 5:
14 × 10 = 140
140 ÷ 2 = 70
Let’s try 23 × 5:
23 × 10 = 230
230 = 220 + 10
Half of 220 + 10 is 110 + 5
110 + 5 = 115
Th ese will become lightning mental calculations after just a few
minutes of practice.
EXPERIMENTING WITH REFERENCE NUMBERS
We use the reference numbers 10 and 100 because they are so easy
to apply. It is easy to multiply by 10 (add a zero to the end of the
number) and by 100 (add two zeros to the end of the number). It
also makes sense because your fi rst step gives you the beginning of
the answer.
We have also used 20 and 50 as reference numbers. Th en we used 5
as a reference number to multiply very low numbers. In fact, we can
use any number as a reference. We used 5 as a reference number to
multiply 6 times 4 because we saw that 10 wasn’t suitable.
Let’s try using 9 as our reference number.