The time ratio is (10/(3+x))/(10/(x-3)) = (x-3)/(x+3).

This problem is mainly about the application of t=s/t formula. Pay attention to the speed difference between downstream and upstream. Downstream or water deceleration, downstream or water acceleration. Then divide the speed by 10km to find the downstream and upstream speeds respectively, and then compare them to get it. If x is required, there are no conditions.