A river 400 m wide flows at a rate of 2.0 m/s . A boat sails at a...

The width of the river is $d = 400 \text{ m}$.

The velocity of the boat with respect to the water is $v_{br} = 10 \text{ m/s}$.

Since the boat sails in a direction perpendicular to the river flow, the component of its velocity along the width of the river is exactly $v_{br}$.

The time taken to cross the river is given by the ratio of the width of the river to the component of the boat's velocity perpendicular to the river flow.

$t = \dfrac{d}{v_{br}}$

$t = \dfrac{400}{10} = 40 \text{ s}$