anujtsingh wrote:

In the morning, John drove to his mother's house in the village at an average speed of 60 kilometers per hour. When he was going back to town in the evening, he drove more cautiously and his speed was lower. If John went the same distance in the evening as in the morning, what was John's average speed for the entire trip?

(1) In the evening, John drove at a constant speed of 40 kilometers per hour.

(2) John's morning drive lasted 2 hours.

We are given that John drives at a rate of 60 km per hour and drives at a lesser rate when driving again later. We also are given that the distances driven are the same and need to determine the average rate.

We can use the formula:

average speed = total distance/total time

Statement One Alone:

In the evening, John drove at a constant speed of 40 kilometers per hour.

Since the distance each way is d, we can let time 1 = d/60, and time 2 = d/40; thus:

average speed = 2d/(d/60 + d/40)

average speed = 2d/(2d/120 + 3d/120)

average speed = 2d/(5d/120)

average speed = 240d/5d = 48

Statement one alone is sufficient to answer the question.

Statement Two Alone:

John's morning drive lasted 2 hours.

Knowing only the total time is not enough to determine the average speed.

Answer: A

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