The ratio of the speed of a boat is that of the current of water as 36:5. The boat goes along with the current in 2 hours 35 minutes. It will come back in?

3 hrs 25 min

ATQ,      Boat speed(x)  :  Current speed(y)

                    36           :      5

Then, to calculate the ratio of downstream to upstream, i.e. 

Downstream = (x + y)

Upsteeam = (x - y)

   (D.S) Downstream     :     (U.S) upstream

          36 + 5               :         36 - 5 

Speed                41              :             31

Time                  31              :             41 (reverse of speed, since distance is same)

                      5x  ↓                              ↓ x5

        (given)   155 min                       205 min