Call the sum S. Then, you can use the definition of percentage to obtain an equation to find the value of S: 1) 28% = 28 / 100; 2) 28% of x = (28/100)x; 3) that is equal to $100.80 => (28/100)x = 100.8 => x = 100.8 * 100 / 28 => x = 360. You can verify that the 28% of 360 is 100.80: 360 * 28/100 = 100.80. Then, the answer is $360.