A boat is sent across a river with a velocity of 8km /h if the resultant velocity of boat is 10km /h then velocity of the river is

Question

$\begin{array}{1 1} (a) \;10\;km/hr \\ (b) \;8\;km/hr\\ (c) \;6\;km/hr \\ (d) \;4\;km/hr\end{array} $