To find the amount that the population increased, we subtract the original population from the new population:

x + y - x = y

We then plug it into a percent equation, which says, what percent of (x + y) is y, which is our increase:

(x + y)*n = y

(x + y)n = y

n = y / (x + y)

Where n is the percent increase, make sure you multiply n by 100 to get the decimal in terms of percentage, which makes your equation this:

n = 100*(y) / (x + y)