The ratio of speed of a motorboat to that of the current of water is 35 : 5. The motorboat goes along with the current in 5 h 10 min. Find the time to come back of motorboat?

Let speed of a motorboat be = 36k km/h
and speed of the current = 5k km/h
∴ Speed downstream = (36 + 5)k = 41k km/h
and speed upstream = (36 - 5)k = 31k km/h
Let distance be D km.
According to the question
When boat goes along with the current.
Distance = Time x Speed
⇒ D = (5 + 10/60) x 41k
⇒ D = 31/6 x 41k ....(i)
Again, when boat come back
a = Time x Speed
From Eq. (i) .
31/6 x 41k = Time x 31k
⇒ Time = 41/6
Time = 6 h 50 min.