Understanding Elapsed Time

Elapsed time is simply the amount of time that passes from the beginning of an event to its end.

Whether you are trying to figure out how long a car ride took, or what time your favorite movie will finish, you are calculating elapsed time!

Finding How Much Time Has Passed

If you know the start time and the end time, you can figure out the total time that has passed by counting the hours and then the minutes.

Example: From 2:15 to 4:45, how much time has passed?

1. Count the hours first: Jump from 2:15 to 3:15 ($1$ hour), then to 4:15 ($2$ hours).
2. Count the minutes next: From 4:15 to 4:45 is $30$ minutes ($45 - 15 = 30$).

Answer: The elapsed time is $2$ hours and $30$ minutes.

Finding the End Time

Sometimes you know when an event starts and how long it lasts, but you need to figure out when it will end.

Example: A movie starts at 7:30 and lasts $1$ hour and $45$ minutes. When does it end?

1. Add the hours: Start at 7:30. Add $1$ hour to get to 8:30.
2. Add the minutes: Now we need to add $45$ minutes to 8:30.
   • First, add $30$ minutes to reach the next hour: 8:30 + $30$ minutes = 9:00.
   • We had $45$ minutes to add, and we just used $30$. We still have $15$ minutes left ($45 - 30 = 15$).
   • Add the remaining $15$ minutes to 9:00 to get 9:15.

Answer: The movie ends at 9:15.

Using a Number Line

A blank number line is one of the best tools for solving elapsed time problems!

• Draw a line and write your Start Time on the left.
• Make large "jumps" forward for the hours.
• Make smaller "jumps" forward for the minutes (you can jump by 10s, 5s, or 1s to make it easier).
• Where you land is your End Time!

Using jumps on a number line helps keep track of the time so you don't lose your place when crossing over into a new hour.