A boat can travel 18 km in 3 hours while going upstream, and 40 km in 4 hours while going downstream. What is the speed of the boat in still water?

8 km/h

We know that,

Upstream Speed = Speed of Boat – Speed of current

Downstream Speed = Speed of Boat + Speed of current

Distance = Speed × Time

Let the speed of the boat in still water and the flow of the river be S and R km/h respectively.

Upstream speed = (S - R) km/h

Downstream speed = (S + R) km/h

According to the concept,

3(S - R) = 18

= (S - R) = 6      ....(1)

4(S + R) = 40

= (S + R) = 10      ....(2)

(1) + (2),

S - R + S + R = 16

= 2S = 16

= S = 8