Try these:
a) 8 × 5 =
b) 4 × 5 =
c) 2 × 5 =
d) 6 × 5 =
The answers are:
a) 40
b) 20
c) 10
d) 30
This is what we do when the tens digit is odd. Let’s try 7 × 5. First multiply by 10:
7 × 10 = 70
If you find it difficult to halve the 70, split it into 60 + 10. Half of 60 is 30 and half of 10 is 5, which
gives us 35.
Let’s try another:
9 × 5 =
Ten nines are 90. Ninety splits to 80 + 10. Half of 80 + 10 is 40 + 5, so our answer is 45.
Test yourself
Try these for yourself:
a) 3 × 5 =
b) 5 × 5 =
c) 9 × 5 =
d) 7 × 5 =
The answers are:
a) 15
b) 25
c) 45
d) 35
This is an easy way to learn the 5 times multiplication table.
This 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
These will become lightning mental calculations after just a few minutes 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 first step gives you the beginning of the answer.
We have also used 20 and 50 as reference numbers. Then 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