A part of a journey is covered in $1$ hour $40$ minutes at $75$ km/hr and the remaining part of it is covered in $2$ hours $50$ minutes at $60$ km/hr. Find the average speed during the whole journey.
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$ 36\cfrac{3}{7} $ km/hr
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$ 65\cfrac{5}{9} $ km/hr
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$ 86\cfrac{4}{3} $ km/hr
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$ 23\cfrac{4}{7} $ km/hr
B
Correct answer
Explanation
Part 1: 1h 40m = 5/3 hours. Distance = 75 * 5/3 = 125 km. Part 2: 2h 50m = 17/6 hours. Distance = 60 * 17/6 = 170 km. Total distance = 295 km. Total time = 5/3 + 17/6 = 10/6 + 17/6 = 27/6 = 4.5 hours. Average speed = 295 / 4.5 = 590 / 9 = 65 and 5/9 km/hr.