A boat goes 10 km upstream and 12 km downstream in 4 hours. It can also go 7.5 km upstream and 7 km downstream in 2.75 hours. What is the speed (in km/h) of the boat when going upstream?
Show answer & explanation
Correct answer: D
Explanation
The correct answer is option 4 i.e. 4.
Given:
A boat goes 10 km upstream and 12 km downstream in 4 hours.
It can also go 7.5 km upstream and 7 km downstream in 2.75 hours.
Formula used:
Time = Distance/Speed ---- (1)
Calculations:
Let the speed of boat and stream be x and y
Speed upstream = x - y
Speed downstream = x + y
According to the question;
⇒ 10/(x - y) + 12/(x + y) = 4 ---- (2)
⇒ 7.5/(x - y) + 7/(x + y) = 11/4 ---- (3)
Solving equation (2) and (3):
⇒ x + y = 8 and x - y = 4
Hence, speed of the boat going upstream = 4 km/h