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# How many hours did it take Helen to drive from her house to her parent's house?

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How many hours did it take Helen to drive from her house to her parent's house?
(1) Helen's average speed on this trip was 72 kilometers per hour
(2) If Helen's average speed on this trip had been 8 kilometers per hour greater , it would have taken her 1 hour less.

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This question is pretty straight forwars, asking for the number of hours it took for helen to drive to her parents house.

(1) tells us that the total distance(D) over the total time (T) = 72km/hr.  However, since we don't know anything else, we cannot determine how long it took her.  For example the total distance could be 72 km, in which case it would have taken her 1 hour.  Or the total distance could be 216 km, in which case it would have taken her 3 hours.

(2) so if her average speed was 8 hours greater, then it would have taken her 1 hour less.  You can put it into an equation like so D/(T-1)=(D/T)-8.  However, there are multiple times for this to work.

If we look at both together, however we know that D/T=72km/hr and that D/(T-1)=80 at this point we can stop and see that we can solve it, so you can just pick C, but I will finish the problem for you anyway :)

D=72T, so 72T/(T-1)=80

80T-80=72T ---> 8T=80, T=10

Hope this helps!

Eliza, GMAT Tutor

answered Jan 14, 2015 by Associate (125 points)
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72 is avg speed. And equatn will be 72 equals to x/t. X is distance and t is total,time, another eq is 80 equals to x/t-1. Now divide these two,eqns., x will,be eradicated

answered Jan 9, 2015 by Partner (696 points)
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