A swimmer wishes to cross a 500 m wide river flowing at 5 km/h. His speed relative to water is 3 km/h. The shortest possible time to cross the river is

When the velocity of swimmer relative to water is perpendicular to the direction of flow of river, the swimmer can cross the river in minimum possible time. 

$\therefore t_{\min }=\frac{(500/1000)}{3}h=\frac{60}{6}\min =10\min$