A car covers a distance of 360 km in 4 hours 30 minutes. Find its average speed. 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 A
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 person walking at 4/5 of his usual speed reaches his office 10 minutes late. What is his usual time?
Correct Answer: Option A
Explanation:
New speed = 4/5 of usual speed => New time = 5/4 of usual time. Difference = 1/4 of usual time = 10 minutes. Usual time = 10 * 4 = 40 minutes.
New speed = 4/5 of usual speed => New time = 5/4 of usual time. Difference = 1/4 of usual time = 10 minutes. Usual time = 10 * 4 = 40 minutes.
This question belongs to:
Maths
Time Speed and Distance
Question #3
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Two trains of equal lengths take 10 seconds and 15 seconds respectively to cross a telegraph post. If the length of each train is 120 meters, in what time will they cross each other traveling in opposite directions?
Correct Answer: Option D
Explanation:
Speed of first train = 120 / 10 = 12 m/s. Speed of second train = 120 / 15 = 8 m/s. Total distance to cross each other = 120 + 120 = 240 meters. Relative speed in opposite directions = 12 + 8 = 20 m/s. Time = 240 / 20 = 12 seconds.
Speed of first train = 120 / 10 = 12 m/s. Speed of second train = 120 / 15 = 8 m/s. Total distance to cross each other = 120 + 120 = 240 meters. Relative speed in opposite directions = 12 + 8 = 20 m/s. Time = 240 / 20 = 12 seconds.
This question belongs to:
Maths
Time Speed and Distance
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