SOLUTION: Clara and John are travelling to San Diego from their home. They are going to be driving separate cars as John has to leave 1.2 hours later. Clara drives at an average of 60 miles

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Question 1124458: Clara and John are travelling to San Diego from their home. They are going to be driving separate cars as John has to leave 1.2 hours later. Clara drives at an average of 60 miles per hour. John on the other hand drives an average of 70 miles per hour. How long in hours will it take until John catches up with Clara?
A) 9 hours
B) 8.4 hours
C) 8.2 hours
D) 7.6 hours
E) 7.2 hours
Found 2 solutions by josgarithmetic, ikleyn:
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
                   SPEED      TIME          DISTANCE

Clara               60         d/60          d

John                70         d/70          d

Difference                      1.2

d%2F60-d%2F70=1.2, and after finding d, evaluate d%2F70.
-
d%2F6-d%2F7=12
d%281%2F6-1%2F7%29=12
d%287-6%29%2F42=12
d%2F42=12
d=12%2A42
-
d%2F70=%2812%2A42%29%2F70=%282%2A2%2A3%2A6%2A7%29%2F%282%2A5%2A7%29
%282%2A3%2A6%29%2F5
36%2F5
highlight%287%261%2F5%29-------------choice E.

Answer by ikleyn(52781) About Me  (Show Source):
You can put this solution on YOUR website!
.
Short solution and the answer 

Time = %2860%2A1.2%29%2F%2870-60%29 = 7.2 hours = 7 hours and 12 minutes.     // Option E).


Explanation 

At the moment John starts, Clara is 60*1.2 miles ahead.


Their relative speed is  70-60 = 10 miles per hour.

It is the rate of decreasing the distance between them.


Therefore,  the ratio  %2860%2A1.2%29%2F%2870-60%29  is the time when John will catch up Clara.