Ok, you'd start off with a square, with four equal sides. Let's assign 100 to be the length of each of these sides. This means the area of the square is 100 * 100 = 10000.
The width of the square is increased by 15%, (turning it into a rectangle), So now you have two sides of 100 and two of 115.
110 * 115 = 11500 is the new area
The change is 11500-10000 = 1500.
As a percentage: 1500/10000 * 100 = 15%