A boat moves downstream at the rate of 1km in 6 minutes and upstream at the rate of 1 km in 10 minutes. The speed of the current (in km/hr) is :

1

4

3

2

Downstream a = 1/(6/60) = 60/6 = 10 km/hr; Upstream b = 1/(10/60) = 60/10 = 6 km/hr. Speed of the current = (a-b)/2 = (10 - 6)/2 = 4/2 = 2 km/hr.

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