A boat travels from point A to B a distance of 12 km From A it travels 4 km downstream in 15 minutes...........

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A boat travels from point A to B, a distance of 12 km. From A it travels 4 km downstream in 15 minutes and the remaining 8 km upstream to reach B. If the downstream speed is twice as high as the upstream speed, what is the average speed of the boat for the journey from A to B?

10(2/3)km/hr

9.6 km/hr

11.16 km/hr

10.44 km/hr

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Explanation:

4 km downstream is covered in 15 min.
∴ speed of downstream = 16 km/hr.
So speed of upstream = 8km/hr.
Total time taken for downstream journey = 15 min (given).
Now total time taken for upstream journey = 8/8 = 1 hr = 60 min.
Hence total time taken from A to B = 15 + 60 = 75 min.
As average speed = Total distance /total time,
so average speed from A to B = (12/75)*60 = 48/5 = 9.6kmph.