question_answer

A boat which has a speed of 5 km/hr in still water crosses a river of width 1 km along the shortest possible path in 15 minutes. The velocity of the river water in \[km/hr\,\]is

A) 3         

B)        4    

C) \[\sqrt{21}\]                  

D)        1

Correct Answer: A

Solution :

[a] Speed along the shortest path \[=\frac{1}{15/60}=\text{4 km/hr}\] Speed of water \[\text{v}=\sqrt{{{5}^{2}}-{{4}^{2}}}=3\text{ km/hr}\]