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A bus travels the first one-third distance at a speed of 10 km/h, the next one-third at 20 km/h and the last one-third at 60 km/h. The average speed of the bus is

Asked in NEET 1991 · Distance and average speed

Answer: (3) 18 km/h

Step-by-step solution

Given: three equal thirds of the distance at 10, 20 and 60 km/h.

Idea: average speed is total distance over total time, and each third takes a different time.

Let the total distance be s, so each leg is s/3.

Total time =s/(30)+s/(60)+s/(180)=(6s+3s+s)/(180)=(10s)/(180)=s/(18).

Average speed =s/(s/18)=18 km/h.

Why the other options are wrong

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