Understanding Elapsed Time Word Problems

Elapsed time is the amount of time that passes from the start of an event to its end. In Grade 4, you will learn how to figure out how long an event lasts, what time it starts, or what time it ends.

Converting Time Units

Before solving problems, it is helpful to remember these basic time conversions:

• $1 \text{ day} = 24 \text{ hours}$
• $1 \text{ hour} = 60 \text{ minutes}$
• $1 \text{ minute} = 60 \text{ seconds}$

Finding the Duration

When you know the start time and the end time, you need to find the duration (how long the event lasted). A great strategy is to "jump" forward on a number line to the next whole hour.

Example: A movie starts at 3:45 PM and ends at 5:30 PM. How long is it?

1. Jump from 3:45 PM to the next hour, 4:00 PM. That is 15 minutes.
2. Jump from 4:00 PM to 5:00 PM. That is 1 hour.
3. Jump from 5:00 PM to 5:30 PM. That is 30 minutes.
4. Add your jumps together: $1 \text{ hour} + 15 \text{ minutes} + 30 \text{ minutes} = 1 \text{ hour and } 45 \text{ minutes}$.

Finding the End Time

If you know the start time and the duration, you add the time to find out when the event finishes. It is usually easiest to add the hours first, then the minutes.

Example: A bus leaves at 8:15 AM and travels for 2 hours and 40 minutes. What time does it arrive?

1. Start at 8:15 AM and add the hours: $8:15 \text{ AM} + 2 \text{ hours} = 10:15 \text{ AM}$.
2. Next, add the minutes: $15 \text{ minutes} + 40 \text{ minutes} = 55 \text{ minutes}$.
3. The arrival time is 10:55 AM.

Finding the Start Time

When you know the end time and the duration, you must work backward (subtract) to find the start time.

Example: A cake finishes baking at 4:10 PM. It took 35 minutes to bake. When did it go in the oven?

1. Start at 4:10 PM. Subtract 10 minutes to get back to the whole hour, 4:00 PM.
2. You still need to subtract 25 more minutes ($35 - 10 = 25$).
3. Subtracting 25 minutes from 4:00 PM gives a start time of 3:35 PM.