A boy covers the first $40$ km of a $60$ km journey at $40$ km/h and the remaining $20$ km at $20$ km/h. His…

A boy covers the first $40$ km of a $60$ km journey at $40$ km/h and the remaining $20$ km at $20$ km/h. His average speed for the whole journey is:
  1. $28$ km/h
  2. $30$ km/h
  3. $32$ km/h
  4. $33\dfrac{1}{3}$ km/h

Solution

First leg $= 40$ km at $40$ km/h $= 1$ hr. Second leg $= 20$ km at $20$ km/h $= 1$ hr. Total $= 60$ km in $2$ hr; average $= 30$ km/h.

Asked in: IMO

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