A driver completed the first 20 miles of a 40mile trip at an average speed of 50 miles per hour. At what average speed must the driver complete the remaining 20 miles to achieve an average speed of 60 miles per hour for the entire 40mile trip? (Assume that the driver did not make any stops during the 40mile trip.)
(A) 65 mph
(B) 68 mph
(C) 70 mph
(D) 75 mph
(E) 80 mph
Answer: D
Source: Official Guide
Solve 700Level Algebra Qs In 90 Secs!
Master 700level Inequalities and Absolute Value Questions
Attend this free GMAT Algebra Webinar and learn how to master the most challenging Inequalities and Absolute Value problems with ease.
A driver completed the first 20 miles of a 40mile trip at an average speed of 50 miles per hour. At what average speed
This topic has expert replies

 Legendary Member
 Posts: 1622
 Joined: Thu Mar 01, 2018 7:22 am
 Followed by:2 members
Timer
00:00
Your Answer
A
B
C
D
E
Global Stats
GMAT/MBA Expert
 [email protected]
 GMAT Instructor
 Posts: 16201
 Joined: Mon Dec 08, 2008 6:26 pm
 Location: Vancouver, BC
 Thanked: 5254 times
 Followed by:1268 members
 GMAT Score:770
Average speed = (total distance)/(total time)Gmat_mission wrote: ↑Wed Feb 23, 2022 8:41 amA driver completed the first 20 miles of a 40mile trip at an average speed of 50 miles per hour. At what average speed must the driver complete the remaining 20 miles to achieve an average speed of 60 miles per hour for the entire 40mile trip? (Assume that the driver did not make any stops during the 40mile trip.)
(A) 65 mph
(B) 68 mph
(C) 70 mph
(D) 75 mph
(E) 80 mph
Answer: D
Source: Official Guide
We already know that the total distance travelled = 40 miles
And we know that we want the average speed to be 60 miles per hour
So, our equation becomes: 60 = 40/(total time)
We can rearrange this equation to get: total time = 40/60 = 2/3 hours
During the first part of the trip, the driver travels 20 miles at a speed of 50 mph
Time to complete first part = distance/rate = 20/50 = 2/5 hours
During the second part of the trip, the driver travels 20 miles at an unknown speed. So let's say that speed is x mph
Time to complete second part = distance/rate = 20/x = 20/x hours
At this point we have enough information to create the following equation: 2/5 + 20/x = 2/3
To eliminate the fractions we'll multiply both sides of the equation by 15x to get: 6x + 300 = 10x
Subtract 6x from both sides to get: to get: 300 = 4x
Solve: x = 300/4 = 75
Answer: D