X, Y are two points in a river. Points P and Q divide the straight line XY into three equal parts. The river flows along XY and the time taken by a boat to row from X to Q and from Y to Q are in the ratio 4 : 5. The ratio of the speed of the boat downstream to that of the river current is equal to:

Let the boat's speed in still water be b and current speed be c. Downstream speed = b+c, upstream speed = b-c. Points divide XY into 3 equal parts, so XQ = 2/3 of XY. Time ratio XQ:YQ = 4:5 means (2XY/3)/(b+c) : (XY/3)/(b-c) = 4:5. Solving gives 2(b-c) : (b+c) = 4:5, so 10b-10c = 4b+4c, giving 6b = 14c, or b/c = 7/3. Therefore (b+c)/c = 10/3.