A person covers half of his journey at 40 km/h, one-third

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A person covers half of his journey at 40 km/h, one-third at 60 km/h and rest of journey at 30 km/h. Find his average speed for the whole journey.?

1. 45 km/h
2. 42. 35 km/h
3. 50 km/h
4. 40 km/h

Correct Option: B

Let total distance be 100 km.
Then, Average = 100/[ (50/40) + (30/60) + (20/30)]
= 100/( 5/4 + 1/2 + 2/3) = 100/[(15 + 6 + 8)/12]
= (100 x 12)/29
= 42.35