Rounding Numbers Using a Number Line

Lesson Plan - Get It!

Audio:

Rounding a number is a way to simplify the number to make it easier to understand.

When rounding a number, you can estimate the value and find a number that it is closest to that ends in a zero. You will use your knowledge of place value to help you round numbers.

Numbers are usually rounded to the nearest tens place or hundreds place. Estimating is a very helpful tool for everyday life. Estimation is commonly used when talking about money amounts, time, the amount of an object, and distances.

If you are at the store and buy three pieces of candy, and each one costs 99 cents, you might say it costs about $3. By rounding 99 cents to a dollar, you can easily add up the amounts instead of counting the exact total.

Watch a short explanation of rounding and using a number line. As you watch the first three minutes of Math Antics - Rounding below, write down the answers to these questions:

1. What does it mean to round a number?
2. According to the video, what are some reasons to round numbers?
3. When would it be best to use an exact number instead of an estimation?

One way to round a number is to use a number line.

Rounding a number means to find another number to which the number is near, like the nearest ten or hundred. When you round numbers, you are finding a value that is good enough to represent the value.

Example 1 Round 68 to the nearest ten.

Since the question asks you to round to the nearest "ten,", find the multiple of ten that is nearest to 68. On the number line, you will see that 68 falls in-between 60 and 70. Since 68 is closer to 70 than to 60, you round up to 70.

Example 2 Round 165 to the nearest ten.

When you round a number to the nearest ten, even if it is a number in the hundreds or thousands, you find the multiple of ten that it is closest to. Since 6 is in the tens place of 165, you will round to 160 or 170.

When the number you are rounding is halfway between two numbers, you usually round up to the larger number. So, 165 would round up to 170.

When estimating, especially with money, it is better to round up and have too much, then round down and not have enough. If you were buying an item that cost about $165, you would need $170 to have enough money. If you estimated and rounded down, you would only have $160, which would not be enough.

Example 3 At the beginning of the lesson, you were asked the following questions:

The lifeguard said there were about 200 people at the pool on Saturday. The manager said there were definitely 175 people at the pool on Saturday.

• Was the lifeguard correct?
• How did he come up with that number?
•  Is it alright to estimate numbers?

When rounding numbers to the nearest hundred, you are finding which multiple of 100 the number is closest to. Multiples of 100 are the numbers you say when you count by 100: 100, 200, 300, 400, and so on.

The number line below shows 100 to 400 and the estimated location of 175. The number 175 falls in-between 100 and 200. The number 150 would be halfway between 100 and 200.

Since 175 is greater than 150, it is closer to 200 than 100, so we round up to 200. The lifeguard was correct because he estimated about 200 people were at the pool.

Consider the following:

• Why do we estimate numbers?
• What happens if a number is halfway in between two values? Do you round up or down? Why would this be important to know?
• Recall a time that you have had to round numbers.

In the Got It? section, you will practice rounding numbers while playing interactive games.