Speed of a boat in standing water is 8 kmph and the speed of the stream is 2 kmph. A man rows to place at a distance of 210 km and comes back to the starting point. The total time taken by him is:  Options.
A) 48 hours
B) 51 hours
C) 36 hours
D) 56 hours
E) none

Rationale: "sol . speed upstream = 6 kmph ; speed downstream = 10 kmph . ∴ total time taken = [ 210 / 6 + 210 / 10 ] hours = 56 hours . answer d"
Answer: D