A boat can go 60 km downstream and 40 km upstream in 12 hours 30 minutes. It can go 84 km downstream and 63 km upstream in 18 hours 54 minutes. What is the speed (in km/h, to the nearest integer) of the boat in still water?
Show answer & explanation
Correct answer: C
Explanation
The correct answer is Option 3 i.e. 9.
Suppose the speed of the boat be x and the speed of the stream be y;
So,
Downstream speed = x + y
Upstream speed = x - y
According to the question:
⇒ 60/(x + y) + 40/(x - y) = 12.5 km/h....(1
⇒ 84/(x + y) + 63/(x - y) = 18.9 km/h ....(2
After solving equations 1 and 2:
⇒ x + y = 40/3
⇒ x - y = 5
So,
x = 9 km/h
Hence, the speed of the boat in still water = 9 km/h