Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles

[BTGmoderatorDC]

Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip

A. 35
B. 40
C. 45
D. 50
E. 55



OA B

Source: GMAT Prep

[Brent@GMATPrepNow]

Susan drove an average speed of 30 miles per hour for the first 30 miles of a trip & then at a average speed of 60 miles/hr for the remaining 30 miles of the trip if she made no stops during the trip what was susan's avg speed in miles/hr for the entire trip

A. 35
B. 40
C. 45
D. 50
E. 55



OA B

Source: GMAT Prep

Average speed = (TOTAL distance)/(TOTAL time)

TOTAL distance
Susan traveled 30 miles at a speed of 30mph, and 30 miles more at a speed of 60mph.
So, TOTAL distance = 30 miles + 30 miles = 60 miles

TOTAL time
TOTAL time = (time spent driving 30 mph) + (time spent driving 60 mph)
time = distance/speed
- Time spent driving 30 mph = 30 miles/30mph = 1 hour
- Time spent driving 60 mph = 30 miles/60mph = 0.5 hours
TOTAL time = (1 hour) + (0.5 hours) = 1.5 hours
----------------------------------

So, Average speed = (TOTAL distance)/(TOTAL time)
= 60 miles/1.5 hours
= 40 mph

Answer: B

Cheers,
Brent