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1. A boat covers 48 km upstream and 72 km downstream in 12 hours, while it covers 72 km upstream and 48 km downstream in 13 hours The speed of stream is :

A. 2 kmph

B. 2.2 kmph

C. 2.5 kmph

D. 4 kmph

Solution:

Let downstream speed be D and Upstream speed be U. Then

\begin{aligned} \frac{48}{U} + \frac{72}{D} = 12 . . . . . . . (1) \\ and \\ \frac{72}{U} + \frac{48}{D} = 13 . . . . . . . (2) \end{aligned}

On solving the equation (1) and (2), we get
D = 12 kmph and
U = 8 kmph
Thus, speed of the current

= \frac{D - U}{2} = \frac{4}{2} = 2 kmph

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