Question 986168
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45 miles per hour times 1/60 hours per minute is 3/4 miles per minute.  In 12 minutes, the #12 train has traveled 3/4 miles per minute times 12 minutes equals 9 miles.  Hence, when the #7 train starts, the distance between the two trains is 13 minus 9 equals 4 miles.


Since the trains are heading toward each other, the speed with which they approach each other is the sum of their speeds, namely 45 plus 35 equals 80 miles per hour.  80 miles per hour times 1/60 hours per minute is 4/3 miles per minute.  So the trains will meet 4 miles divided by 4/3 miles per minute equals 3 minutes after the #7 train starts.


In 3 minutes the #12 train will travel 3/4 miles per minute times 3 minutes = 9/4 miles.  Hence, the total distance traveled by the #12 train is the 9 miles traveled before the #7 train started and the 9/4 miles traveled after the #7 train started:  9 plus 9/4 = 11 and 1/4 miles.


The #12 train left the station 12 minutes before the #7 train started and an additional 3 minutes after the #7 train started for a total of 15 minutes. 


John
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My calculator said it, I believe it, that settles it

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