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?
Answer & explanation
Correct answer: option 2
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