Question

The width of a river is $5m$ and in it, water is flowing with a speed of $4m/min$. A boatman is standing on the bank of the river. He wants to sail the boat to a point at the other bank which is directly opposite to him, In what time will he cross the river, if he can sail the boat at $8m/min$, relative to the water?

• A. $0.72min$
• B. $3min$
• C. $3.6min$
• D. $5.2min$

Answer 1

The correct option is A $0.72min$

$\begin{array}{cc}{v}_{net}& =\sqrt{8^2-4^2}\\ & =\sqrt{64-16}\\ & =\sqrt{48}\\ t=& \frac{d}{v_{net}}=\frac{5}{\sqrt{48}}\\ & =0.72min\end{array}$