A tank contains 10,000 gallons of a solution that is 5% sodium chloride by volume. If 2,500 gallons of water evaporate from the tank, the remaining solution will be about what percent sodium chloride?

My solution equation is as follows:

0.95(x + 0.05) = 0.05(10,000 + 7500x

This equation led to the wrong answer.

Why is this equation wrong?