The speed of boat in still water is 25 km/hr and the speed of current is 7 km/hr. The boat goes from place A to place B and return from B to A and it takes 6 hours 40 minutes in total. Find the time taken by boat to cover twice the distance of AB downstream.

4 hrs 48 min

(D.S) Downstream speed = 25 + 7 = 32 km/hr

(U.S) Upstream speed = 25 - 7 = 18 km/hr

Distance between A to B = \(\frac{D.S\;×\;U.S}{D.S\;+\;U.S}\) × Time

                                      =  \(\frac{32\;×\;18}{32\;+\;18}\) × 6\(\frac{2}{3}\)

                                      = \(\frac{32\;×\;18}{50}\) × \(\frac{20}{3}\)

                                      = \(\frac{32\;×\;12}{5}\) km

Time to cover twice distance = \(\frac{32\;×\;12\;×\;2}{32\;×\;5}\) = 4 hours 48 minutes