SOLUTION: Hi Henry and Harry drove from a to b 12km away. Henry's average speed was 3km per hour faster than Harry's speed. If Henry reached b 8 minutes faster than Harry, what is Harry's

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Question 1186166: Hi
Henry and Harry drove from a to b 12km away. Henry's average speed was 3km per hour faster than Harry's speed. If Henry reached b 8 minutes faster than Harry, what is Harry's average speed.
Thanks

Answer by ikleyn(52794) About Me  (Show Source):
You can put this solution on YOUR website!
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Henry and Harry drove from a to b 12km away. Henry's average speed was 3km per hour faster than Harry's speed.
If Henry reached b 8 minutes faster than Harry, what is Harry's average speed.
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Let x be the slower speed, in km/h.

Then the faster speed is (x+3) km/h.


Now you write the "time" equation


    12%2Fx - 12%2F%28x%2B3%29 = 8%2F60  of an hour.    (1)


Simplify it


    12%2Fx - 12%2F%28x%2B3%29 = 2%2F15

    %2812%2A3%29%2F%28x%2A%28x%2B3%29%29 = 2%2F15

    %286%2A3%29%2F%28x%2A%28x%2B3%29%29 = 1%2F15

    x*(x+3) = 15*18.


From this point, you  GUESS  the  ANSWER  MENTALLY  and momentarily:  x = 15 km/h.


ANSWER.  Harry's average speed is 15 km/h.


CHECK.  12%2F15 - 12%2F18 = %2812%2A6%29%2F90+-+%2812%2A5%29%2F90 = 12%2F90 = 8%2F60,   exactly as in equation (1).  ! Correct !

Solved.