Estimate 7.83 × 42.1 ÷ 3.9

Step by step

The question

Question: estimate 7.83 × 42.1 ÷ 3.9.

To estimate means to find about how big the answer is.

We make every number easy first.

1 Round to 1 significant figure

We round each number to 1 significant figure: only its first figure stays.

7.83: the first figure is 7.

The next digit is 8, 5 or more, so we round up: 8.

42.1: the first figure is 4.

The next digit is 2, 4 or less, so we round down: 40.

3.9: the first figure is 3.

The next digit is 9, 5 or more, so we round up: 4.

2 Work it out

So the answer is about 8 × 40 ÷ 4.

Now it is easy to work out.

Careful!

We changed the numbers.

So we write ≈, about, not =.

8 times 40 is 320.

320 divided by 4 is 80.

So the answer is about 80.

3 Check

Let's check with a calculator.

The exact answer is about 84.52.

That is close to 80.

So the estimate is good!

Let's look back.

First: round to 1 significant figure.

Then: work it out.

Last: check.