Date: 02/27/2001 at 17:55:49 From: alyssa wilson Subject: Working backward word problem A marathon runner crossed the finish line after running for 2 hours, 48 minutes, and 12 seconds. She crossed the line at 9:18:12 A.M. At what time did the marathon start? I did it this way. 9 9:18:12 - 2:48:12 my answer was 6:70 minutes. I don't get this. My teacher said the answer was 6:30.
Date: 02/28/2001 at 12:15:14 From: Doctor Peterson Subject: Re: Working backward word problem Hi, Alyssa. It sounds as if you tried to subtract as if these were just ordinary numbers: 91812 - 24812 ------- 67000 But in working with time, place value is a little different. The 9 stands for the number of hours, not the number of ten-thousands; and one hour is 60 minutes, not 100 minutes. That changes the way you "borrow" (regroup) when you subtract. Here's one way to subtract times: First subtract the seconds, 12 - 12 = 0. Then subtract the minutes: 18 - 48 = ? We can't do it, so we have to borrow from the hours. That means we take one hour (changing the 9 to 8) and turn it into 60 minutes (adding that to the 18). Now we can subtract 48 from 78 and get 30. Here's the work: 8 7 / / 9:18:12 - 2:48:12 --------- 6:30:00 That gives your teacher's answer. If you didn't quite follow that, here are some other explanations that I found by searching our archives for "subtract time": Adding and Subtracting Units of Time http://mathforum.org/dr.math/problems/shellie12.11.98.html Calculating Timecards http://mathforum.org/dr.math/problems/felker.8.3.99.html - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
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