Hi

A boat crossed a lake from North to South at the speed of 4 km/h, entered a river and covered twice as much distance going upstream at 3 km/h. It then turned around and stopped at the south shore of the lake. If it averaged 3.8 km/h that day, what was its approximate downstream speed?I'm a bit unsure about answer explanation in this, I think the answer is correct but we don't need to calculate anything at all, the question itself gives the answer, here is why :

Speed in lake (which can taken as speed in "still water", as happens in such problems) = 4 km/h and upstream speed = 3 km/h, so speed of river = 4-3 = 1 km/h.

Naturally, downstream speed = speed in "still water" + speed of river = 4 + 1 = 5 km/h

Opinions ?

Regards,

Subhash

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