Elapsed Time
Learn to calculate how much time has passed between two events.
Definition
Try it now
Worked Examples
Emma started reading at 3:15 PM and finished at 3:45 PM. How long did she read?
Identify the start and end times
Start: 3:15 PM, End: 3:45 PM → Both times in the same hour
Subtract the minutes
→ 30 minutes
Check: the hour stayed the same
Both are 3 o'clock → No hours to add
Answer: Emma read for 30 minutes.
Common Mistakes
Subtracting minutes incorrectly across hours
Why it's wrong: Students often forget that there are 60 minutes in an hour, not 100. When subtracting 3:15 from 4:10, they might say 55 minutes instead of the correct answer.
Correct: Remember: 1 hour = 60 minutes. When subtracting, you may need to borrow 60 minutes from the hour.
Confusing AM and PM when crossing noon or midnight
Why it's wrong: Students forget that noon (12:00) marks the change from AM to PM, and midnight marks PM to AM.
Correct: Draw a timeline showing the full day. Remember: 12:00 PM is noon (daytime), 12:00 AM is midnight (nighttime).
Adding instead of subtracting (or vice versa)
Why it's wrong: Students confuse whether to add or subtract when finding start time vs end time vs elapsed time.
Correct: To find elapsed time: subtract. To find end time: add. To find start time: subtract from end time.
Interactive Visual
Elapsed Time
Interactive Sandbox
Expression Calculator
Try these:
History
No calculations yet
Practice Problems
15 problemsA movie started at 3:00 PM and ended at 5:00 PM. How long was the movie?
Why It Matters
- Planning activities: Knowing how long homework or sports practice will take
- Cooking: Timing recipes correctly (bake for 45 minutes)
- Travel: Figuring out how long a trip will take
- Schedules: Knowing when you need to leave to arrive on time
- TV and movies: Understanding how long until your favorite show ends
Real World Applications
Planning Your Day
We use elapsed time to plan activities and manage schedules efficiently.
Example:
If your music lesson starts at 4:30 PM and lasts 45 minutes, you know it ends at 5:15 PM, so you can plan what to do after.
School ends at 3:20 PM. You have soccer practice at 5:00 PM. It takes 25 minutes to walk home and 15 minutes to walk to the field.
How much free time do you have at home?
Step 1: Write the mathematical expression
Calculate your free time:
Cooking and Baking
Recipes often require precise timing for cooking or baking.
Example:
A recipe says to marinate chicken for 2 hours. If you start at 11:45 AM, the chicken will be ready to cook at 1:45 PM.
You need to make a meal with three steps: prep vegetables (20 min), cook rice (25 min), and bake chicken (55 min). The rice and chicken can cook at the same time, but prep must be done first.
If dinner needs to be ready at 6:30 PM, what time should you start?
Step 1: Write the mathematical expression
Calculate total time needed:
Travel Planning
Calculating travel time helps us arrive on time for appointments and events.
Example:
If a flight departs at 2:30 PM and takes 3 hours 45 minutes, it arrives at 6:15 PM.
Your family is driving to the beach. You leave at 8:15 AM and arrive at 11:50 AM.
How long was the drive?
Step 1: Write the mathematical expression
Calculate the elapsed time:
Key Takeaways
- 1Elapsed time is the amount of time that passes between a start time and an end time
- 2To find elapsed time: subtract the start time from the end time
- 3To find the end time: add the elapsed time to the start time
- 4To find the start time: subtract the elapsed time from the end time
- 5Remember: 1 hour = 60 minutes, not 100!
- 6Use a number line or count by hours then minutes to avoid mistakes
Frequently Asked Questions
Glossary
- Elapsed time
- The amount of time that passes from the start of an event to the end
- Start time
- The time when an event or activity begins
- End time
- The time when an event or activity finishes
- Duration
- Another word for how long something lasts (same as elapsed time)
- AM
- Times from midnight (12:00 AM) to just before noon (11:59 AM)
- PM
- Times from noon (12:00 PM) to just before midnight (11:59 PM)