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Question

A car travels from A to B at a speed of 20 km/h and returns at a speed of 30 km/h. The average speed of the car for the whole journey is

A
25 km/h
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B
24 km/h
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C
50 km/h
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D
5 km/h
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Solution

The correct option is B 24 km/h
While going from A to B, let the speed be V1 and from B to A the speed be V2.
V1=20 km/h
V2=30 km/h

Average speed v=TotaldistanceTotaltime ---(i)
If d be the distance AB. Then we can say:
Time required for A to B=t1=dV1
Time required for B to A=t2=dV2
From eq. (i),
v=d+dt1+t2
v=2ddV1+dV2
v=2V1V2V1+V2

v=2×20×3020+30
v=24 km/h


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