Question 200669: roy drives to work at 70 mph and home that night along the same route at 30 mph.
what is his average speed ? Answer by vleith(2983) (Show Source):
You can put this solution on YOUR website! Careful here. The kneejerk answer is (70+30)/2 = 50
But is not correct.
Why?
Sure the distance both way is the same, but the time required to cover that distance is less going 70 than it is at 30.
Let the distance be given as D
The total distance is 2D.
So we need to find out how much time is spent on the trip each way
So
Thus, becaue the return trip takes over twice as long as getting to work, the average speed is well less than the kneejerk answer.
Make sure you understand this concept. A problem just like this was on every SAT/GRE test I ever took.
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