4th Grade Math: Help with Elapsed Time
In 4th grade, students learn to solve word problems involving units of measurement, including elapsed time. For an explanation of this concept, as well as some sample problems, read on!
Math Problems with Elapsed Time
Elapsed time is the amount of time that passes between one point of time and another. Just like you can measure the distance between two points in inches or meters, you can measure the time between two points in seconds, minutes or hours.
Solving Elapsed Time Problems
Addition
The most basic problems will ask you to find the time between two points using addition. For instance, you might get a problem like this one:
Carrie left school at 3:15 p.m., and she returned home one hour and 20 minutes later. What time did she get home?
To solve this problem, begin by adding one hour to the start time. One hour after 3:15 p.m. is 4:15 p.m., so Carrie got home 20 minutes after 4:15. To get your final answer, just add 15 minutes and 20 minutes to get 35 minutes (15 + 20 = 35). Carrie arrived home at 4:35 p.m.
Subtraction
You may also solve problems that ask you to find the difference between two times, like this one:
Samantha started her homework at 5:15 p.m. and finished it at 8:05 p.m. If she worked the entire time, how much time did she spend on her homework?
To solve a problem like this, you'll need to figure out how many hours and minutes passed between these two times. One way to solve this problem is by working backwards. If the answer was three hours, then Samantha would have finished working at 8:15 p.m. Instead, she finished working ten minutes before 8:15, which means the answer is ten minutes less than three hours, or two hours and 50 minutes.
Multiplication and Division
You may also solve elapsed time problems involving multiplication or division. Some problems will give you the amount of time a particular task takes, and then ask you to figure out how long it would take you to perform the task a certain number of times.
For example, if it takes you one minute and five seconds to do 20 pushups, how long would it take you to do 40 pushups? The answer would be 1:05 x 2, but you can't multiply time the same way you can multiply whole numbers. Instead, multiply the number of minutes by two (1 x 2 = 2) and add that to the number of seconds multiplied by two (5 x 2 = 10). Then put them back together to get your answer, two minutes and ten seconds.
You may also perform the reverse operation. Imagine that you knew you could do 40 pushups in two minutes and ten seconds, and you were asked how many pushups you could do in one minute and five seconds at the same rate. You would divide two minutes and ten seconds each by two to get your answer (one minute and five seconds).
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