So the river is 2.5 feet deep, and when you find that 9% is, using the cross-multiplication formula to find the percent then 9% of 2.5 is 0.225, and when you add that to 2.5 you get 2.725, and when you find 6% of 2.725, you get 0.1635, and since it dropped, you subtract, and you end up with 2.5615