Q: A swimmer moving upstream passes a float at point P. After two hour, he returns and at this instant it meets the float again at a distance 15 km from the point P. If the swimmer velocity in water remains constant, then find the speed of river flow.

(a) 5 kmh^{-1}

(b) 10 kmh^{-1}

(c) 3.75 kmh^{-1}

(d) 7.5 kmh^{-1}

Ans: (d)

Solution:Let Speed of river flow is v

$\displaystyle v = \frac{distance}{time}$

$\displaystyle v = \frac{15}{2}$

v = 7.5 km/h

As swimmer velocity in water remains constant, the speed of river flow is 7.5 km/h