algebra word problem

Question

Jim's motorboat travels downstream at the rate of 15km/h. Going upstream it travels at 7km/h. Write an equation that expresses each fact. What is the rate of the current.

Answer 1

Let x = speed of current and let S = speed of the boat in still water

Speed downstream = x + S = 15

Speed upstream = S - x = 7

Adding the equations, we get 2S = 22. So, S = 11.

Therefore, x + 11 = 15, so x = 4 km/hour