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Question
A boy travelled half the distance from his house to school at a speed of 10 km/hr. For exactly half of the remaining time he travelled at 15 km/hr and the rest of the time at 20 km/hr. Find his average speed. Options: A) 13 km/hr B) 140/11 km/hr C) 15 km/hr D) 97/6 km/hr
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Answer
Let the total distance be 2d.
Total time = d/10 + x/15 + (d-x)/20
x/15 = (d-x)/20 => x = 3d/7
Total time = d/10 + 3d/105 + 4d/140 = 11d/70
Average speed = Total distance/Total time = 2d/(11d/70) = 140/11 kmph
