Deepthi Subbu wrote:A car travels from point A to point B. The average speed of the car is 60 miles/hr and it travels the first half of the trip at a speed of 90 mi/hr. What is the speed of the car in the second half of the trip?
30 ,
45,
60,
75 ,
cannot be determined
This is essentially a weighted average question. No real math is needed.
The faster speed is 90mph.
The average speed for the whole trip is 60 mph.
The slower speed must be less than the average of 60mph. Eliminate C, D and E.
Looking at answer choice A, we can see that (30+90)/2= 60. But if the slower speed were 30mph, the average speed for the whole trip would be
less than 60mph, because more time would be spent traveling at 30mph than at 90mph. Eliminate A.
The correct answer is
B.
Another approach would be to plug in a distance.
Let d = 180.
Time for first half = d/r = 90/90 = 1 hour.
Time for whole trip = d/r = 180/60 = 3 hours.
Thus, time for second half = 3-1 = 2 hours.
Rate for second half = d/t = 90/2 = 45mph.
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