Problem number 15. Warren is on a treadmill in the gym. He keeps looking at two stopwatches. The first shows the time elapsed since he started the session, and the second, the time remaining until the end of his session. At some point, the two stopwatches show the same reading. What did they show at that point? To solve this problem, notice that the first stopwatch shows the time from the beginning to the current point in time, and the second will show the time from the current point in time to the end. That is, if we add the two stopwatches together, the elapsed time plus the time left, we will get the total amount of time from the start of the session to the end of the session. And this total time is the exact same no matter what time it is currently, because that is a constant total amount of time Warren is in the gym. In this case, we know that that total time is 14 minutes 58 seconds plus 21 minutes 32 seconds, which is what the stopwatches currently show. And that equals 35 minutes 90 seconds, or 36 minutes and 30 seconds. Now, if the two stopwatches showed the same reading, we know that both of them have to add together to be 36 minutes and 30 seconds. So that means that the halfway point is when we divide 36 minutes and 30 seconds, the total amount of time, by 2, which will be 18 minutes and 15 seconds. And in that case, both of the stopwatches will show 18 minutes 15 seconds, which add up to the total time of 36 minutes and 30 seconds. So that means the answer will be D, 18 minutes 15 seconds.

treadmill

stopwatches

elapsed time

time remaining

session total