Mathematics Essentials Q1

Rounding Decimals Using a Number Line

Let’s think back for a minute to rounding whole numbers. When we were rounding whole numbers, we could round a number to any place value that we wanted to. We could round to tens, hundreds, thousands, etc.

To do this, we followed a few simple rules.

1. Look at the digit to the right of the place value you are rounding.
2. If the digit to the right is a five or greater, you round up.
3. If the digit to the right is less than 5, you round down.

Example

Round to the nearest ten, 46

The four is in the tens place, that is the place we are rounding. The six is in the ones place that is the digit we look at. Since 6 is a five or greater, we round up.

46 becomes 50. Our answer is 50.

There are a couple of different ways that we can round decimals.

First, let’s look at rounding them using a number line.

Here we have a number line. You can see that it start with zero and ends with one. This number line has been divided up into quarters.

It goes from 0 to .25 to .50 to .75 to 1.0.

Example

.33

Here we have .33. The first thing that we want to do is to graph it on a number line.

We want to round to the nearest quarter. This number line gives us a terrific visual to do this.

Which quarter is .33 closest to? It is closest to .25.

Our answer is .25.

We can also round decimals to the nearest whole using a number line.

Example

Round 4.2 to the nearest whole.

Here we can use our number line to show us which whole number 4.2 is closest too.

Is 4.2 closer to 4.0 or 5.0 on the number line? It is closer to 4.0.

Our answer is 4.0.