Question

A river $4.0$ miles wide is flowing at the rate of $2miles/h$. The minimum time taken by a boat to cross the river with a speed $V=4miles/h$ (in still water) is approximately

• A. $1h$ and $9$ minutes
• B. $2h$ and $7$ minutes
• C. $1h$ and $12$ minutes
• D. $2h$ and $25$ minutes

Answer 1

The correct option is A $1h$ and $9$ minutes

The net speed of the boat = $v_b=\sqrt{(4{)}^2+(2{)}^2}=2\sqrt{5}miles/h$

time taken = distance / speed = 4 miles/ $2\sqrt{5}miles/h$

=$2/\sqrt{5}h$