Question 1200887
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The times for the two parts of the trip were the same, so the ratio of distances for the two parts is the same as the ratio of speeds.<br>
The ratio of the speeds is 10:15 = 2:3, so 2/5 of the trip was at 10 mph and 3/5 was at 15 mph.  Since the total trip was 100 miles, the first part of the trip was 40 miles and the second part was 60 miles.<br>
The times for the two parts of the trip were 40/10 = 4 hours and 60/15 = 4 hours, so the total time for the trip was 8 hours.<br>
ANSWER: 8 hours<br>