Two outposts M and N are 320 km apart. Train A leaves M for N at 40 km/h at 6:00 AM. Train B leaves N for M at 60 km/h at 8:00 AM. At what time will they cross?
A man covers 3/5 of his journey at 60 km/h, 1/4 of his journey at 40 km/h and the remaining journey at 20 km/h. Find his average speed for the whole transaction.
Explanation:
Let total distance be 400 km. First part = 240 km; Time = 240/60 = 4 hours. Second part = 100 km; Time = 100/40 = 2.5 hours. Remaining distance = 400 - 240 - 100 = 60 km; Time = 60/20 = 3 hours. Total time = 4 + 2.5 + 3 = 9.5 hours. Average speed = 400 / 9.5 = 4000 / 95 = 46.15 km/h.
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