The speed of a swimmer in still water is 20m s ^ - 1 . The speed of...

For reaching just apposite bank of the river the net velocity at the river should be zero that means

$vsin\left(\theta\right) = velocityofriver$

$sin\theta = \dfrac{velocityofriver}{velocityofswimmer}$

$sin\text{\theta} = \dfrac{10}{20} = \dfrac{1}{2}$
$\text{\theta} = 30 °$
$30 °$ west of north