MathRevolution wrote:

[GMAT math practice question]

Tom traveled the entire 90 km trip. If he travelled the first 18 km of the trip at a constant rate of 36 km per hour and the remaining trip at a constant rate of 72 km per hour, what is his average speed?

A. 30 km/h

B. 36 km/h

C. 45 km/h

D. 48 km/h

E. 60 km/h

Average speed = \(\frac{TotalDistance}{TotalTime}\)

We need to find the time for each part of the trip (which requires easily finding distance for second part), add the times, and divide into total distance of 90.

1) TIME for Part 1 of trip? RT=D: He traveled 18 km at 36 km per hour. Distance/rate = time, so he took

\(Time=\frac{D}{R}: \frac{18}{36}=\frac{1}{2}hr\)2) DISTANCE for Part 2? 90 km total, 18 km covered, means (90 - 18) = 72 km remain

3) TIME, Part 2? He traveled 72 km at a speed of 72 km per hour, so

\(Time=\frac{D}{R}: \frac{72}{72}= 1 hr\)4) AVERAGE SPEED?

Total distance = 90 km

Total time:

\((\frac{1}{2} + 1)hrs = \frac{3}{2} hrs\)Average speed in km per hr:

\(\frac{90}{(\frac{3}{2})}=(90 * \frac{2}{3})=60\)Answer E

_________________

In the depths of winter, I finally learned

that within me there lay an invincible summer.

-- Albert Camus, "Return to Tipasa"