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Question

A car travels first half distance with uniform speed of 60 km/hr. What should be its uniform speed (in km/hr) for the second half of the distance so that its average speed over the entire journey becomes 90 km/hr.

A
120
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B
150
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C
180
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D
210
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Solution

The correct option is C 180
Let the total distance travelled by the car over entire journey be 2D.
For first half distance D:
Time taken=DistanceAverage speed=D60
For the latter half distance D, let the average speed be v
Time taken =Dv
For total journey:
Total distance=Average speed ×Total Time
2D=90×(D60+Dv)
D45=D60+Dv
1v=145160
v=180 km/h

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