Boats and Streams Practice Questions

5 questions per page • Detailed step-by-step solutions

Question 16
Easy
A boat goes 12 km downstream and comes back to the starting point in 3 hours. If the speed of the current is 3 km/hr, then the speed (in km/hr) of the boat in still water is :
A 8 km/h
B 6 km/h
C 12 km/h
D 9 km/h
Correct Answer: D

Explanation:

12/(u + 3) + 12/(u - 3) = 3 => 4/(u + 3) + 4/(u - 3) = 1 => 8u / (u^2 - 9) = 1 => u^2 - 8u - 9 = 0 => (u - 9)(u + 1) = 0 => u = 9 km/h.
Question 17
Medium
A boat covers 24 km upstream and 36 km downstream in 6 hours, while it covers 36 km upstream and 24 km downstream in 6 1/2 hours. The speed of the current is:
A 1.5 km/hr
B 2.5 km/hr
C 1 km/hr
D 2 km/hr
Correct Answer: D

Explanation:

24/U + 36/D = 6 and 36/U + 24/D = 6.5. Solving gives U = 8 km/hr and D = 12 km/hr. Speed of current = (12 - 8) / 2 = 2 km/hr.
Question 18
Easy
The speed of a boat along the stream is 8 km/h and against the stream is 6 km/hr. The time taken by the boat to sail 28 km in still water is
A 2 hrs
B 3 hrs
C 4 hrs
D 8 hrs
Correct Answer: C

Explanation:

Downstream speed = 8 km/h, Upstream speed = 6 km/h. Boat speed in still water = (8 + 6) / 2 = 7 km/h. Time to sail 28 km = 28 / 7 = 4 hours.
Question 19
Easy
A man can row 7.5 kmph in still water, if the river is running at 1.5 km an hour. It takes him 50 mins to row to a place and back, how far off is the place?
A 1 km
B 2 km
C 3 km
D 4 km
Correct Answer: C

Explanation:

Downstream speed = 7.5 + 1.5 = 9 kmph. Upstream speed = 7.5 - 1.5 = 6 kmph. Total time = 50 min = 5/6 hr. D/9 + D/6 = 5D/18 = 5/6 => D = 3 km.
Question 20
Easy
Speed of stream is 7 km/hr and downstream speed is 35 km/hr. Find the speed of upstream?
A 36 km/hr
B 21 km/hr
C 96 km/hr
D 27 km/hr
Correct Answer: B

Explanation:

Speed of boat in still water u = Downstream - Stream = 35 - 7 = 28 km/hr. Upstream speed = u - v = 28 - 7 = 21 km/hr.