Speed of a boat in standing water is 9 kmph and the speed of the stream is 6 kmph. A man rows to place at a distance of 170 km and comes back to the starting point. The total time taken by him is:	Choose the correct option.
A) 12 hours  B) 68 hours  C) 36 hours  D) 58 hours  E) none

Rationale: "sol . speed upstream = 3 kmph ; speed downstream = 15 kmph . ∴ total time taken = [ 105 / 7.5 + 105 / 10.5 ] hours = 68 hours . answer b"
Answer: B