Danny drove his old car 100 kilometers...

[AAPL]

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 complete 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.

I don't have clear this PS question, I think that I can solve it of the following way,

I know that the total distance is 100 kilometers, Danny should stop for 10 minutes every 40 kilometer and his average speed when he's moving is 60 kilometers per hour, then average speed will be,
$$A_{speed}=\frac{Total\ Dist}{Total\ Time}$$
Total time will be,

40 minutes (moving) + 10 minutes (first stop) + 40 minutes (moving) + 10 minutes (second stop) + 20 minutes (rest of distante) = 120 minutes or 2 hours.
$$A_{speed}=\frac{100\ kilometers}{2\ hours}=50\ kilometers\ per\ hour$$
I appreciate if any expert explain it for me. Thank you so much.

[ErikaPrepScholar]

Your explanation nails it! Solving for total distance over total time is a guaranteed way to get the correct answer on average speed problems.

One other thought on solving this problem: you should be able to eliminate D and E right off the bat: Danny never drives faster that 60 kph, so there is no possible way that his average speed could be greater than 60 kph.

[Jay@ManhattanReview]

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 complete 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.

I don't have clear this PS question, I think that I can solve it of the following way,

I know that the total distance is 100 kilometers, Danny should stop for 10 minutes every 40 kilometers and his average speed when he's moving is 60 kilometers per hour, then average speed will be,
$$A_{speed}=\frac{Total\ Dist}{Total\ Time}$$
Total time will be,

40 minutes (moving) + 10 minutes (first stop) + 40 minutes (moving) + 10 minutes (second stop) + 20 minutes (rest of distante) = 120 minutes or 2 hours.
$$A_{speed}=\frac{100\ kilometers}{2\ hours}=50\ kilometers\ per\ hour$$
I appreciate if any expert explain it for me. Thank you so much.

Hi AAPL,

You did it fine, though it could have been crisp.

His average speed when he's moving is 60 kilometers per hour = 60 kilometers per 60 minutes = 1 kilometer per minute

Thus, to cover 100 kilometers, he would take 100 minutes. During the 100 km drive, he would take two breaks of 10 minutes each. Thus, total time taken = 100 + 2*10 = 120 minutes

Average speed of the trip = Distance/Time = 100/120 = 5/6 km/min = (5/6)*60 = 50 km/hour

The correct answer: C

Hope this helps!

-Jay
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[Jeff@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 complete 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

Since we need to calculate average speed, we can use the equation:

Average speed = total distance/total time

We are given that Danny drove his old car 100 kilometers, from his home to a friend, and that he stopped his car for 10 minutes every time he completed 40 kilometers of continuous driving. Since he drove 100 miles, he stopped his car twice for a total of 20 minutes or 20/60 = 1/3 hour.

We are also given that Danny drove at an average speed of 60 kilometers per hour. Since time = distance/rate, Danny's actual driving time for the 100 kilometers was 100/60 = 5/3 hours.

Finally we can determine his average speed for the entire trip.

Average speed = total distance/total time

Average speed = 100/(1/3 + 5/3)

Average speed = 100/(6/3) = 100/2 = 50 kilometers per hour

Answer: C