Question
A person travels from town A to town B at 40 km/h and
returns from B to A at 60 km/h. Find his average speed for the whole journey.Solution
Let distance A to B = D km. Time from A to B = D/40 Time from B to A = D/60 Total distance = 2D Total time = D/40 + D/60 = D(1/40 + 1/60) LCM(40, 60) = 120 1/40 + 1/60 = 3/120 + 2/120 = 5/120 = 1/24 Total time = D Γ 1/24 = D/24 Average speed = Total distance / Total time = 2D / (D/24) = 2D Γ (24/D) = 48 km/h
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