Story Problems: Elapsed Time

Lesson Plan - Get It!

Audio:

The amount of time between two events is called elapsed time.

Elapsed time is the difference between two times, or the amount of time that passes.

First, watch a short explanation of calculating elapsed time by using a number line. As you watch Elapsed Time: How to Solve Elapsed Time on a Number Line, from Tools for Teachers by Laura J., write down the answers to these questions:

1. What does elapsed time mean?
2. When using a number line, at what time would you start the number line?
3. How many minutes were in-between each hour on the number line?

To calculate elapsed time, find the difference in time between the earlier time and the later time. One way to calculate elapsed time is with a number line. Another way is to form a subtraction equation like this:

Later time - earlier time = difference in time

Example 1 How many years were there from 1955 to 1990?

Method 1 Number line

Elapsed time can be counted in years, day, hours, and even minutes. Since this example is years, the number line is labeled in years by 10s. Since the first year is 1955, it falls halfway in-between 1950 and 1960. It is 5 years from 1955 to 1960, then you can count by 10s to reach 1990. Add up the amount of time that passed — 5 + 10 + 10 + 10 = 35 years.

Method 2 Subtraction equation

Later time - earlier time = difference in time

1990 - 1955 = 35 years

Example 2 The football game started at 3:30 pm and ended at 5:55 pm. How long did the football game last?

Below is a number line that shows the game started at 3:30 and ended just before 6:00 pm at 5:55 pm. You can count by hours from 3:30 to 4:30 and 4:30 to 5:30. If you counted another hour from 5:30 to 6:30, that would go past the ending time of 5:55 pm. So, count the minutes from 5:30 to 5:55, which is 25 minutes. 1 hour + 1 hour + 25 minutes = 2 hours and 25 minutes.

To solve this problem using subtraction, subtract the later time and the earlier time to find the difference in time.

The football game lasted 2 hours and 25 minutes.

It is important to remember that when working with time, every 60 minutes equals 1 hour. If the minutes are 60 or more in your solution, subtract 60 from the minutes and add 1 to the total hours.

• For example, if the difference from subtraction was 2:75, that would mean 2 hours and 75 minutes. 75 minutes is greater than 60 minutes, so you need to convert the 75 minutes into hours and minutes. 75 - 60 = 15, so add one more hour to your time to rewrite your solution as 3 hours and 15 minutes, instead of 2 hours and 75 minutes.

Consider the following:

• Why is it important to be able to calculate elapsed time?
• What two ways can be used to help you calculate elapsed time?

In the Got It? section, you will practice calculating elapsed time while completing interactive practice.