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Venkat can row a boat in still water at the speed of 12 km/h. He ferries tourists 15 km upstream and 18 km downstream in 3 hours. Find the speed of the stream.
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Sol. Let speed of the stream be $x$ km/h
$\frac{15}{12 - x} + \frac{18}{12 + x} = 3$ (I) (2)
$\Rightarrow x^2 - x - 12 = 0$ (II) (1)
$\Rightarrow (x - 4)(x + 3) = 0$ (III) (1)
$\Rightarrow x = 4, -3$
x = -3 (rejected)
$\therefore x = 4$ (IV) (1)
The speed of the stream = 4 km/h
$\frac{15}{12 - x} + \frac{18}{12 + x} = 3$ (I) (2)
$\Rightarrow x^2 - x - 12 = 0$ (II) (1)
$\Rightarrow (x - 4)(x + 3) = 0$ (III) (1)
$\Rightarrow x = 4, -3$
x = -3 (rejected)
$\therefore x = 4$ (IV) (1)
The speed of the stream = 4 km/h