Distance/Rate Problems

[swerve]

Danny drove his old car 100 kilometers, from his home to a friend. To prevent the engine from overheating, he stopped his car for 10 minutes every time he completed 40 kilometers of continuous driving. When the car was moving, Danny drove it at an average speed of 60 kilometers per hour. What was Danny's average speed on that trip?

A. 37.5 kilometers per hour
B. 48 kilometers per hour
C. 50 kilometers per hour
D. 75 kilometers per hour
E. 100 kilometers per hour

The OA is C

Source: Economist GMAT

[jbryant62]

Danny drove his old car 100 kilometers, from his home to a friend. To prevent the engine from overheating, he stopped his car for 10 minutes every time he completed 40 kilometers of continuous driving. When the car was moving, Danny drove it at an average speed of 60 kilometers per hour. What was Danny's average speed on that trip?

A. 37.5 kilometers per hour
B. 48 kilometers per hour
C. 50 kilometers per hour
D. 75 kilometers per hour
E. 100 kilometers per hour

The OA is C

Source: Economist GMAT

Since he drives at 60km/h, each mile takes one minute. he drove 100km therefore he drove for 100minutes.

Now every 40km he stopped for 10 minutes so, he drove 40, stopped 10, drove 40 stopped 10, drove the last 20. So he stopped for a total of 20 minutes.

100+20=120 minutes.
He drove 100km in 2 hours or 50km per hour.

[Scott@TargetTestPrep]

Danny drove his old car 100 kilometers, from his home to a friend. To prevent the engine from overheating, he stopped his car for 10 minutes every time he completed 40 kilometers of continuous driving. When the car was moving, Danny drove it at an average speed of 60 kilometers per hour. What was Danny's average speed on that trip?

A. 37.5 kilometers per hour
B. 48 kilometers per hour
C. 50 kilometers per hour
D. 75 kilometers per hour
E. 100 kilometers per hour

The OA is C

Source: Economist GMAT

Excluding the time spent during the breaks, Danny traveled 100 kilometers at an average speed of 60 km/h; thus his time is 100/60 = 5/3 hours. Since he takes a 10 minute break after driving 40 kilometers, he will stop two times during the 100 kilometer trip; thus he will spend 10 + 10 = 20 minutes, or 20/60 = 1/3 hours during the breaks. Thus, including the time spent during the breaks, it took Danny 5/3 + 1/3 = 6/3 = 2 hours to travel 100 kilometers. Then, his average speed for the whole journey is 100/2 = 50 km/h.

Answer: C