zz0vlb wrote:
How many hours did it take Helen to drive from her house to her parents's house?
(1) Helen's average speed on this trip was 72 km/hr.
(2) If Helen's average speed on this trip has been 8 km/hr greater, it would have taken her 1 hour less.
Source: GMAT Prep
Can someone explain this clearly.
Hi,
Its C.
You need to calculate the distance, let that be D.
Now from 1 you just get the speed. To know D you need time and speed both. so 1 is insufficient
Using 2 you just get the difference in speed and the time, again no concrete values. so 2 is insufficient.
However if you combine 1 and 2 you get 2 speeds and the difference in time.
Here is how the solution would be
Earlier speed = 72.
time taken at 72 km/hr = t1
t1 = D/72
Later speed = 72+8 = 80 km/hr
Time taken at 80 km/hr = t2
t2 = D/80
Now t1-t2 = 1
D/72 -D/80 = 1
Solve this and you get D = 720 km.