A boat can row 36 km upstream in 9 hours and the same distance downstream in 3 hours. What is the speed of the boat in still water ?

8 km/hr

We know that,

Speed = Distance/Time

We have,

D = 36 km

Time taken upstream = 9 hours

Time taken downstream = 3 hours

Let's denote the boat's speed in still water = B (km/h), and the speed of the current = C (km/h).

When the boat is rowing upstream =

Relative speed = B - C

B - C = \(\frac{36}{9}\)

B - C = 4 ----------- (x)

When the boat is rowing downstream:

B + C = \(\frac{36}{3}\)

B + C = 12 ----------- (y)

Adding Equation x and Equation y, we get:

B - C + B + C = 4 + 12

2B = 16

B = 8