If the first one-third of a journey is travelled at 20 km h - 1 , next one-third at 40 km h - 1 and the last one-third at 60 km h - 1 , then the average speed for the whole journey will be

Question

If the first one-third of a journey is travelled at 20 km h - 1 , next one-third at 40 km h - 1 and the last one-third at 60 km h - 1 , then the average speed for the whole journey will be, 32.7 km h - 1, 35 km h - 1, 40 km h - 1, 45 km h - 1

MARKS App · Accepted Answer

Let t 1 , t 2 , t 3 be the time taken in travelling distances s 3 , s 3 , s 3 . Then t 1 = s / 3 v 1 = s / 3 2 0 , t 2 = s / 3 v 2 = s / 3 4 0 , t 3 = s / 3 v 3 = s / 3 6 0 ∴ Average speed = s s / 3 2 0 + s / 3 4 0 + s / 3 6 0 = 3 1 2 0 + 1 4 0 + 1 6 0 = 3 6 + 3 + 2 1 2 0 = 3 × 1 2 0 1 1 = 32.7 km/h

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