A car travelled first third of a distance at 10 km/h, second third of the distance at 20 km/h and last third of the distance at 60 km/h. Mean speed of the car over entire distance is
A
30 km/h
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B
20 km/h
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C
18 km/h
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D
54 km/h
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Solution
The correct option is C 18 km/h v=dt=dt1+t2+t3
for first part of travelling; 10=d3t1t1=d3×10
for second part of travelling; t2=d3×20
for third part of travelling; t3=d3×60
mean speed (v)=dd3(110+120+160)v=18 km/h
Hence, v= 18 km/h