Mathematics C level: interrest rates percentages - need help! > The number of fish in a lake declined by 30% over 13 years. What is the average yearly decline in the number of fish? I know how to increase, but I can't seem to figure out how to calculate decline, can anyone help explain?

If you really think,$$V=U(1+r)^n$$ where $V$ is final quantity, $U$ is initial quantity (i.e., $1$ in this case), $r$ is the annual rate of change (increase or decrease) and $n$ is the no. of years in this case.

Putting $V=1-0.3=0.7$ and $n=13$, you get $r=-0.02706357116766167$.

That is, a decrease of $2.7\%$ every year.