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The speed of a boat going upstream is 40% of its speed going downstream. It takes a total time of 42⁄3 hours in going 15 km upstream and th…

3221Medium DSSSB TGT Computer Science 0 replies

The speed of a boat going upstream is 40% of its speed going downstream. It takes a total time of 42⁄3 hours in going 15 km upstream and the same distance downstream. What is the speed (in km/h) of the boat in still water?

  • A83⁄8
  • B77⁄8
  • C71⁄2
  • D81⁄2
Correct answer: B

Explanation

Let downstream speed = d, so upstream speed = 0.4d. 15/(0.4d) + 15/d = 14/3 gives 52.5/d = 14/3, so d = 11.25 km/h and upstream = 4.5 km/h. Speed in still water = (11.25 + 4.5)/2 = 7.875 = 7 7/8 km/h.
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