A boat can go 5 km upstream and 7km downstream in 45 minutes. It can also go 5 km downstream and 2.5 km upstream in 25 minutes. How much time (in minutes) will it take to go 6 km upstream?
Show answer & explanation
Correct answer: B
Explanation
The correct answer is option 2 i.e. 36
Speed during upstream = a km/hr
Speed during downstream = b km/hr
According to the question,
5/a + 7.5/b = 3/4.........(1)
5/b + 2.5/a = 25/60 = 5/12......(2)
On multiplying the second equation by 2 we get,
10/b + 5/a = 10/12...........(3)
Subtract eq 1 from 3 we get,
2.5/b = 1/12
b = 30 km/hr
On putting the value of b in eq 1 we get,
a = 10 km/hr.
Required time = 6/10 × 60 = 36 minutes.