7th Grade Math | Percent of Change and Percentages First, calculate the difference between 30 and 15, which is 15 (30 - 15 = 15). Next, divide 15 (the difference in the number of tickets sold) by 15 (the number of tickets sold last week). Since 15/15 = 1, and 1 x 100 = 100, Ellen's ticket sales increased by 100% since last week. 2.