A person row a boat 24 km upstream in 4 hours. If the speed of the current is 2 km/h, find the speed of the boat in still water. MCQ with Answer and Explanation
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Question #1
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A motorboat covers 12 km upstream and returns back to the starting jetty in 3 hours. If the speed of the boat in still water is 9 km/h, find the speed of the river current.
Correct Answer: Option D
Explanation:
Let the speed of the current be c km/h. 12 / (9 - c) + 12 / (9 + c) = 3. Dividing by 3: 4 / (9 - c) + 4 / (9 + c) = 1 => 4(9 + c + 9 - c) = 81 - c^2 => 72 = 81 - c^2 => c^2 = 9 => c = 3 km/h.
Let the speed of the current be c km/h. 12 / (9 - c) + 12 / (9 + c) = 3. Dividing by 3: 4 / (9 - c) + 4 / (9 + c) = 1 => 4(9 + c + 9 - c) = 81 - c^2 => 72 = 81 - c^2 => c^2 = 9 => c = 3 km/h.
This question belongs to:
Maths
Time Speed and Distance
Question #2
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A carriage driving in a fog passed a man who was walking at the rate of 3 km/h in the same direction. He could see the carriage for 4 minutes and it was visible to him up to a distance of 100 meters. What is the speed of the carriage?
Correct Answer: Option A
Explanation:
Time = 4 minutes = 4/60 = 1/15 hour. Distance = 100 meters = 0.1 km. Relative speed = Distance / Time = 0.1 / (1/15) = 1.5 km/h. Since the carriage moves in the same direction, Relative Speed = Speed of carriage - Speed of man => 1.5 = Speed of carriage - 3 => Speed of carriage = 4.5 km/h.
Time = 4 minutes = 4/60 = 1/15 hour. Distance = 100 meters = 0.1 km. Relative speed = Distance / Time = 0.1 / (1/15) = 1.5 km/h. Since the carriage moves in the same direction, Relative Speed = Speed of carriage - Speed of man => 1.5 = Speed of carriage - 3 => Speed of carriage = 4.5 km/h.
This question belongs to:
Maths
Time Speed and Distance
Question #3
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A person runs a distance of 1200 meters along a straight road in 4 minutes. What is his speed in km/h?
Correct Answer: Option A
Explanation:
Time = 4 minutes = 240 seconds. Speed = 1200 / 240 = 5 m/s. Speed in km/h = 5 * (18/5) = 18 km/h.
Time = 4 minutes = 240 seconds. Speed = 1200 / 240 = 5 m/s. Speed in km/h = 5 * (18/5) = 18 km/h.
This question belongs to:
Maths
Time Speed and Distance
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