A man takes 13 hours to swim 91 km downstream. He swims 39 km upstream in the same time. What is the speed (in km/hr) of the boat in still water?​

Correct option is A

Given:

The time taken to swim 91 km downstream = 13 hours.

The time taken to swim 39 km upstream = 13 hours.

The man is swimming in a boat, and we need to find the speed of the boat in still water.

Formula Used:

Speed downstream: boat speed  + current speed  (since the boat and the current are working together)

Speed upstream: boat speed - current speed  (since the current is against the boat)

Speed = $$\frac{\text{Distance}}{\text{Time}}$$​​

Solution: ​

Let speed of boat be b and speed of current be c

Downstream speed (b + c ) = $$\frac{91}{13} = 7 \, \text{km/h}$$​​

Upstream stream (b − c) = $$\frac{39}{13} = 3 \, \text{km/h}$$​ 

adding both equations 

we get 

2b = 10

b = 5 km/h

Thus, the speed of the boat in still water is 5 km/h