On a recent trip, Mary drove 50 miles. What was the average speed at which she drove the 50 miles?

[Gmat_mission]

On a recent trip, Mary drove 50 miles. What was the average speed at which she drove the 50 miles?

(1) She drove 30 miles at an average speed of 60 miles per hour and then drove the remaining 20 miles at an average speed of 50 miles per hour.
(2) She drove a total of 54 minutes.

Answer: D

Source: Official Guide

[Brent@GMATPrepNow]

On a recent trip, Mary drove 50 miles. What was the average speed at which she drove the 50 miles?

(1) She drove 30 miles at an average speed of 60 miles per hour and then drove the remaining 20 miles at an average speed of 50 miles per hour.
(2) She drove a total of 54 minutes.

Answer: D

Source: Official Guide

Target question: What was Mary's average speed for the 50-mile trip?
Average speed = (total distance)/(total travel time)
= (50 miles)/(total travel time)

As you can see, we need only determine the total travel time in order to answer the target question.

Statement 1: She drove 30 miles at an average speed of 60 miles per hour and then drove the remaining 20 miles at an average speed of 50 miles per hour.
For both parts of her journey, we COULD determine the travel time, so we COULD find the total travel time, which means we COULD calculate Mary's average speed
Since we COULD answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: She drove a total of 54 minutes.
Perfect! Her total travel time is 54 minutes, so we COULD calculate Mary's average speed
Since we COULD answer the target question with certainty, statement 2 is SUFFICIENT

Answer: D

Cheers,
Brent