Level 1: Estimate differences
When rounding a number to the nearest ten, look at the ones digit.
If the digit is below 5 (0, 1, 2, 3 or 4) we round down.
If the digit is 5 or higher (5, 6, 7, 8 or 9) we round up.
Have a look at how the numbers below have been rounded up and down:
21 - Round down to 20
42 - Round down to 40
53 - Round down to 50
74 - Round down to 70
35 - Round up to 40
16 - Round up to 20
97 - Round up to 100
68 - Round up to 70
89 - Round up to 90
Let's practise estimating maths problems. Here's an example.
Question
Which subtraction problem has a difference of about 40?
a) 57 - 15 b) 78 - 49
Solution:
First, round each number to the nearest ten:
a) 57 - 15
57 rounded to the nearest ten is 60
15 rounded to the nearest ten is 20
Now subtract:
60 - 20 = 40
b) 78 - 49
78 rounded to the nearest ten is 80
49 rounded to the nearest ten is 50
Now subtract:
80 - 50 = 30
The correct answer is a). The sum is about 40.
