How_To_Be_Good_At_Math

CALCULATING • SUBTRACTION 89

Subtracting as finding the difference

We can also think of subtraction as finding the difference

between two numbers. When we are asked to find the

difference, we are really just finding how many steps it

takes to count from one to the other.

All we have to do is find 3 on So, 8 − 3 = 5

the number line and see how

many jumps it takes to get to 8.

It takes 5 jumps.

To find the difference between

two numbers, we can count up a

number line. Let’s take another look

at the calculation 8 − 3.

0 321 4 5 6 7 8 9 10

8 − 3 =?

To subtract 3 from 8, first So, 8 − 3 = 5

we find 8, then count back

3 places. This takes us to 5.

Look at the calculation 8 – 3

on this number line.

Subtracting as counting back

One way to think of subtraction is called counting back.

When we subtract one number from another, we are just

counting back from the first number by a number of steps

that’s equal to the second number.

0 1 32 4 5 6 7 8 9 10

Start at 8

-1 -1 -1

8 − 3 =? 8 − 3 = 5

8 − 3 = 5

We start at 3, the

second number in

our calculation

Then we count how many

places we have to move to

reach the first number

1 + 1 + 1 + 1 + 1 = 5

In total, we have counted back

3 places, bringing us to 5

088_089_Subtraction.indd 89 29/02/2016 18:02