Estimate Calculations By Rounding to One Significant Figure

In this activity, we will be using our rounding skills to estimate the answers to calculations.

This skill is useful for when we need to make approximate calculations for when we don't have a calculator.

Engineers use this skill to estimate the quantity of materials needed for a building.

We also use rounding when trying to find out what our shopping bill may be as we are going around the supermarket!

Example 1

Round off the values in the calculation to 1 significant figure to work out an estimate to the calculation below.

7.91 + 8.3 + 10.6

First, let's round each value to 1 sig. fig.

7.91: the 1 significant figure we need is the 7. We look at the digit before the 7, which is a 9. This means that it rounds the 7 up to 8.

8.3: the 1 significant figure we need is the 8. We look at the digit before the 8, which is a 3. This means that it rounds down to keep it as 8.

10.6: the 1 significant figure we need is the 1. We look at the digit before the 1, which is a 0. This means that it rounds down to keep it as 1. However, we have to include a zero as a place holder because the number 1 is not the same as the number 10!

This gives us:

7.91 → 8

8.3 → 8

10.6 → 10

Now we have our rounded values, we can find an estimate for the calculation.

8 + 8 + 10

= 26

Example 2

Round off the values in the calculation to 1 significant figure to work out an estimate to the calculation below.

(7.91 + 8.3 ) ÷ 10.1

First, let's round each value to 1 sig. fig.

7.91 → 8

8.3 → 8

10.1 → 10

Now we have our rounded values, we can find an estimate for the calculation.

(8 + 8) ÷ 10

= 16 ÷ 10

= 1.6

Great ... let's try the activity!