John Has A Boat That Will Travel At The Rate Of 15 Kph In Still Water. He Can Go Upstream For 35 Km In The Same Time It Takes To Go 140 Km Downstream. How Fast Is His Boat Traveling When He Goes Downstream?

His boat is traveling 15 kph relative to the stream.

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Assume the speed of the stream is "s." The travel times can be equated like this
  35/(15 - s) = 140(15 + s)
Cross multiplying, we get
  35*(15 + s) = 140*(15 - s)
  35*15 + 35s = 140*15 - 140s    (use the distributive property)
  s(35 + 140) = 15(140 - 35)    (add 140s - 35*15 to both sides, factor)
  s = 15*105/175 = 9

John's boat is traveling 9+15 = 24 kph when it goes downstream.