Elapsed time is the amount of time that has passed between two events. You can calculate the elapsed time by figuring out the difference between the start and stop time.
Here is an elapsed time problem.
Soccer practice begins at 3:15 p.m. and ends at 4:45 p.m. Determine how long soccer practice lasts.
To solve this problem, set up a subtraction problem that could be used to find the number of hours and minutes that pass between those two times.
Subtract the starting time from the stopping time. Remember that both times represent the number of hours and minutes past noon.
4:45− 3:15−−−−−−1:30
Soccer practice lasts for 1 hour 30 minutes.
Here is another problem.
A dance performance starts at 7:30 p.m. and ends at 10:10 p.m. How long does the performance last?
Subtract the starting time from the stopping time.
10:10 −7:30−−−−−−−
You cannot subtract 30 minutes from 10 minutes, so you must rewrite 10:10. Remember, 10:10 stands for 10 hours 10 minutes. Borrow from the hour unit. 1 hour equals 60 minutes. Add 60 minutes to the 10 minutes.
109− 7 2 :1070: 30: 40
Events can also occur before noon (a.m.) and end after noon (p.m.). When that happens, finding the elapsed time requires an additional step. This is a result of using a 12-hour clock.
Here is an example.
John goes to school at 8:00 a.m. and returns home at 3:00 p.m. How long was John at school?
You might think that the elapsed time is 5 hours, which would be incorrect. Before finding the difference, add 12 hours to 3:00 p.m.
12:00 + 1:4013:40
Remember that the day starts at midnight. 8:00 a.m. is 8 hours into the day and 3:00 p.m. is actually 15 hours into the day. Rewrite 3:00 p.m. as 15:00 and find the elapsed time.
15:00 − 8:007:00
John was at school for 7 hours.
ExamplesExample 1
Earlier, you were given a problem about James reading his book.
James started at 4:00 p.m. and stopped at 8:30 p.m. Find the elapsed time between the start and end time.
Subtract the start time from the end time.
8:30− 4 :00 4:30
James spent 4 hours 30 minutes reading.
