Also if you are tight on time you can quickly narrow down the answers to A and B without doing much math.

The non-weighted average of 20 and 40 is 30. Now we know 30 would be too "easy" for the GMAT and also a bit of a trap since 30 is in the problem. So it likely won't be 30.

It will take Charlie longer to cover the distance when he is traveling at a slower speed. So he will spend more time at the slower speed. This will drag down the non-weighted average of 30, so the true (weighted) average will be less than 30. So we can narrow down the answers to A and B.

Take care,
Sionainn Marcoux
BA - Stanford University, MPP - Harvard University
Instructor, tutor for Princeton Review and Airbnb host
In other words a blend of Jamie Escalante from Stand and Deliver, Julie from The Love Boat, and Schneider the Super from One Day at a Time.

M7MBA wrote:Charlie drives 30 miles each way to work and back. If on his drive to work he averages 40 miles per hour, but on his return trip he can only manage an average of 20 miles per hour, what is his average rate for the day, in miles per hour?

(A) 22.5

(B) 26.67

(C) 30

(D) 32.5

(E) 33.33

We can use the formula for average rate:

average rate = total distance/total time

We know that the total distance is 30 x 2 = 60 miles. Since time = distance/rate, the time to work is 30/40 = 3/4 hour and the time from work is 30/20 = 3/2 hours. Thus,

average rate = (30 + 30)/(3/4 + 3/2)

average rate = 60/(3/4 + 6/4) = 60/(9/4) = 240/9 = 26.67

Answer: B