how do i solve this for x

y = x/(x+z)

really bugging me. Is it possible. If you need more information on it i can explain what I'm trying to work out.

If i have an amount which i will call z and i want to add another amount to it which i will call x but i want the amount x to be a certain percentage of x + z. The certain percentage is y represented as a number less than 1.

.25 = 5/(5+15)

25%

i want to be able to take the percentage and the original amount and find out what x need to be to match the original amount to the percentage y.

Any help would be much appreciated.