QUESTION

CSAT

Hard

Maths

Prelims 2020

A car travels from a place X to place Y at an average speed of VV km/hr, from Y to X at an average speed of 2v2v km/hr, again from X to Y at an average speed of 3v3v km/hr and again from Y to X at an average speed of 4v4v km/hr. Then the average speed of the car for the entire journey

Select an option to attempt

Explanation

Formula for Average Speed: Average Speed = Total Distance / Total Time

Total Distance: Total Distance = 4d4d (as there are 4 legs of the journey)

Total Time:

Time for the first leg (X to Y) = d/Vd / V Time for the second leg (Y to X) = d/(2V)d / (2V) Time for the third leg (X to Y again) = d/(3V)d / (3V) Time for the fourth leg (Y to X again) = d/(4V)d / (4V) Total Time = (d/V)+(d/2V)+(d/3V)+(d/4V)(d/V) + (d/2V) + (d/3V) + (d/4V)

Simplifying Total Time: Total Time = d/V(1+1/2+1/3+1/4)=d/V25/12d/V * (1 + 1/2 + 1/3 + 1/4) = d/V * 25/12

Average Speed: Average Speed = 4d/(d/V25/12)=(4d12V)/(25d)=48V/251.92V4d / (d/V * 25/12) = (4d * 12V) / (25d) = 48V / 25 \approx 1.92V

The average speed lies between V and 2V.

Thus, Option B is the correct answer.

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