*Contributor: Ashley Nail. Lesson ID: 13273*

What is elapsed time? Why is it so important in emergency situations? Learn how to calculate elapsed time using a number line and the true story from the movie, *Breakthrough*.

categories

subject

Math

learning style

Visual

personality style

Lion, Otter, Beaver

Grade Level

Intermediate (3-5), Middle School (6-8)

Lesson Type

Quick Query

The movie, *Breakthrough*, is based on a true story about three friends who fell through the ice into a cold lake in Missouri. Watch the beginning clip of *Breakthrough | The Heroes | 20th Century FOX*:

- Did you hear the first responders in the ambulance ask how long the kid has been under the icy water?

The 911 dispatchers respond with "5 to 7 minutes."

This is an example of elapsed time. Elapsed time is the difference between two readings on a clock. A reading at the beginning of an event and another reading after an amount of time has passed.

Think of a stopwatch. When someone begins to run, you start the stopwatch. When they cross the finish line, you stop the stopwatch. The stopwatch shows the elapsed time, or how much time it took the runner to go from the starting line to the finish line.

The same thing can be done with a regular clock, except the time doesn't start at zero like a stopwatch. The elapsed time is found by looking at two readings on a clock and finding the difference.

In an emergency situation like that in *Breakthrough*, it is important for first responders to quickly communicate the elapsed time.

Let's say that when the 911 call was made, the caller's watch looked like this:

First, we need to determine the time on the watch.

We look at the hour hand and notice it is a little past 10, so the time is 10:_ _

Now, we look at the minute hand and count how many minutes it represents. We know that between each number on the clock is 5 minutes, so we can skip count by 5 until we reach the number closest to the minute hand. Then we count by ones.

The minute hand is on 47, so the time is 10:47.

If this is the watch the caller looked at when he called 911, the time of the accident was at 10:47 am.

Then, the 911 dispatcher called the first responders to give instructions. The clock in the police station that the 911 dispatcher looked at may have shown this time:

Looking at the hour hand and the minute hand, we can determine what time it is now.

It is 10:53 am.

Just like in the movie clip, we can now figure out the elapsed time or how long the boy had been in the water.

Let's put our times on a number line, so we can see the time passed in between.

The space in between the two times represents the elapsed time. For this example, all we need to do is count the minutes between the two times.

The elapsed time is 6 minutes! At this point, the boy was underwater for 6 minutes because that's how much time has passed between the two times on the clock.

Now, let's try another example that is a little more challenging.

Look at the two digital clocks.

We will find the elapsed time by putting the times on a number line.

In this example, we cannot just count the minutes between the two times. That would take a long time!

The first time is at 4:51 pm. We need to count how many minutes it takes to get to the next hour.

It takes 9 minutes. Now, we can count how many hours.

There are 2 whole hours between the two times. Now, we need to count the minutes past 7.

There are 15 minutes after 7. Now. we add everything up between the two times on the number line.

2 hours + 9 minutes + 15 minutes

The elapsed time is 2 hours, 24 minutes!

Now, you are ready to practice finding more elapsed time in the *Got It?* section.