How_To_Be_Good_At_Math

92 CALCULATING •^ SUBTRACTING WITH A NUMBER LINE

When you use a number line

for subtraction, it doesn’t

matter if you count down

from the first number or

up from the second number,

the answer will be the same.

Subtracting with

a number line

We have already seen that a number line can help us

with simple subtraction. If we use what we know about

partitioning, we can also use a number line to tackle

more difficult calculations.

Counting back

Let’s use a number line for

132 – 54. To make it easy to

move along the line, we’re going

to partition 54 into three parts.

Starting from 132, we count

back 2 to 130. Next, we

move 50 by making 5 jumps of

10 each, taking us to 80. Finally,

we move another 2 places.

70 80 90 100 110 120 130 140

In all, we’ve moved

54 places, and we’ve

arrived at 78. So, 132 − 54 = 78

(^78132)
Counting up
Remember, we can also
subtract by counting up. This is
called finding the difference.
Let’s look again at 132 − 54.
This time, we’re going to
start at 54, the second
number in our subtraction
calculation, and count up until
we get to the first number, 132.
(^6) +
50
First, we count up 6 places
to 60. Then we take 7 jumps
of 10, and finally another jump
of 2. In all, we’ve moved 78
places. So, 132 − 54 = 78
80 90 100 110 130
132
60 70 120 140
54
In all we move 54 places Start at 132
Start at 54 and count to the right
(^10) + (^10) + (^10) + (^10) + (^10) + (^10) + (^10) + 2 = 78
Stop counting at 132
(^2) + (^10) + (^10) + (^10) + (^10) + (^10) + (^2) = 54
132 − 54 =? 132 − 54 = 78
092_093_Subtracting_with_a_number_line_Shopkeepers_addition.indd 92 29/02/2016 14:10