A person goes downstream at x km/h and upstream at y km/h by the same boat. What is the ratio of speed of boat in still water to speed of water?

Let speed of boat in still water be b and speed of water (current) be c. Downstream speed = b + c = x and upstream speed = b - c = y. Adding: 2b = x + y, so b = (x+y)/2. Subtracting: 2c = x - y, so c = (x-y)/2. Therefore the ratio b : c = (x+y)/2 : (x-y)/2 = (x+y) : (x-y).