Question
A man travels from A to B at a speed of 45 km/h and
returns from B to A at a speed of 90 km/h. Find his average speed for the whole journey.Solution
Let one-way distance = D km. Time from A to B = D/45 Time from B to A = D/90 Total distance = 2D Total time = D/45 + D/90 = D(1/45 + 1/90) LCM(45, 90) = 90 1/45 = 2/90 So 1/45 + 1/90 = 2/90 + 1/90 = 3/90 = 1/30 Total time = D × 1/30 = D/30 Average speed = Total distance / Total time = 2D / (D/30) = 2D × 30/D = 60 km/h Answer: 60 km/h.
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