The answer is simply 25, right?

Take x=K/y, where K is the const. of proportionality. So xy=K.

x(1+25/100)*y(1-d/100)=K. where d is percentage decrease.

(1+25/100)*(1-d/100)=1. dividing both sides by K

Solve it to get d=20, which is the answer.

If x and y are inversely proportional, we can set up an equation like

$y=\frac{n}{x}$

When $x$ is increased by 25%, we now have the equation

$y-m=\frac{n}{1.25x}$

Our expression for percent difference can go as:

$\frac{y-(y-m)}{y} = \frac{m}{y}$

We can replace the variables with the expressions we identified earlier.

We start off with our raw expression for finding the percent decrease.

$\frac{(\frac{n}{x}-\frac{n}{1.25x})}{(\frac{n}{x})}$

We give both fractions of the numerator a common denominator of $1.25x$ and combine like terms such that it becomes

$\frac{(\frac{0.25n}{1.25x})}{(\frac{n}{x})}$

$=(\frac{0.25n}{1.25x})(\frac{x}{n})$

$=\frac{0.25}{1.25} = 0.2 = 20$ %

The percent decrease is $\boxed{20}$ .