Estimate By Rounding to One Significant Figure

Let's say we wanted to guess how many fans would fit into a football stadium.

We might guess 50,000, or 80,000, or 100,000.

We wouldn't usually guess 53,817.

This is because we are only interested in a rough idea of the amount, not an exact value.

When estimating we often use significant figures.

Have a look at this number: 63,849.

The 6 is the first significant figure, the 3 is the 2nd and so on, with the 9 being the 5th significant figure. 

This is because the first digit usually tells us the most information about the size of the number.

In the above example, the 60,000 is more significant than the 800.

Let's look at another: 246,085

It has 6 significant figures.

We must still count the zero as significant this time, as it is in the middle of the number. If we didn't count it, the place value of the other digits would change. 

When we're estimating numbers, a general rule to follow is to always round to 1 significant figure.

Example:

Estimate the following calculation:

578 + 3,202

We round each number to 1 significant figure i.e. after the 1st digit:

578 = 600

3,202 = 3,000

600 + 3,000 = 3,600

578 + 3,202 is approximately 3,600

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