A boat can row 12 km/h in still water. If the river current flows at 3 km/h, find the time taken to row 45 km downstream. MCQ with Answer and Explanation

A boat can row 12 km/h in still water. If the river current flows at 3 km/h, find the time taken to row 45 km downstream.
A. 4 hours
B. 3.5 hours
C. 3 hours
D. 2.5 hours
Answer: Option C
Solution (By JKSSB Mock Tests)
Downstream speed = 12 + 3 = 15 km/h. Time taken = Distance / Downstream speed = 45 / 15 = 3 hours.

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Practice More Time Speed and Distance Questions

Question #1
A car covers three successive 5 km stretches at speeds of 10 km/h, 20 km/h and 30 km/h respectively. Find its average speed.
A. 18.00 km/h
B. 20.00 km/h
C. 15.42 km/h
D. 16.36 km/h

Correct Answer: Option D


Explanation:
Total distance = 3 * 5 = 15 km. Total time taken = 5/10 + 5/20 + 5/30 = 1/2 + 1/4 + 1/6 = (6 + 3 + 2) / 12 = 11/12 hours. Average speed = 15 / (11/12) = 180 / 11 = 16.36 km/h.

This question belongs to: Maths Time Speed and Distance
Question #2
Two trains 140 meters and 160 meters long are running at the speeds of 60 km/h and 40 km/h respectively in opposite directions on parallel tracks. What is the time taken by them to cross each other completely?
A. 12 seconds
B. 10.8 seconds
C. 10 seconds
D. 9 seconds

Correct Answer: Option B


Explanation:
Total distance to be covered = 140 + 160 = 300 meters. Relative speed = 60 + 40 = 100 km/h = 100 * (5/18) = 250/9 m/s. Time = Distance / Speed = 300 / (250/9) = 300 * 9 / 250 = 10.8 seconds.

This question belongs to: Maths Time Speed and Distance
Question #3
A sonar signal takes 6 seconds to travel down to an underwater reef and reflect back to the vessel. If the speed of sound in water is 1500 m/s, find the depth of the reef in kilometers.
A. 4.5 km
B. 5.0 km
C. 4.8 km
D. 4.2 km

Correct Answer: Option A


Explanation:
Total travel time to reef and back = 6 seconds. Time to reach reef = 3 seconds. Depth = Speed * Time = 1500 * 3 = 4500 meters = 4.5 km.

This question belongs to: Maths Time Speed and Distance