A motor boat can travel at 10 Km/hr in still water. It traveled 91 Km downstream in a river and then returned taking altogether 20 hrs. find the rate of flow of river in Km/hr?

3

Speed of boat in still water i.e. x = 10 km/hr

Speed of stream/current i.e. y = ?

Downstream speed = 10 + y

Upstream spead = 10 - y 

According to the question:

 \(\frac{ 91}{\text{10 + y} }\) +  \(\frac{ 91}{\text{10 - y} }\) = 20

In these type of questions, where quadratic equations form it is always better to solve the questions by the options.

Put all the options and see which value of y from the options satisfy the above equation.

So, the answer is 3