The current of a stream runs at 1 km/h. A motorboat goes 35 km upstream and back again at the starting point in 12 h. What is the speed of motorboat in still water?

Let the speed of boat in still water be V km/h. Then,
35/(V - 1) + 35/(V + 1) = 12
⇒ 35(V + 1 + V - 1) = 12(V² - 1)
⇒ 6V² - 35V - 6 = 0
⇒ (V - 6)(6V + 1) = 0
⇒ V = 6 (V = - 1/6 is not possible)
So, speed of boat in still water is 6 km/h.