Their original price was 3.50, meaning that 3.50 is 100% of the price. We can find the percentage of the new price, 2.80, by using ratios.
3.50 100%
2.80 x%
x = 100% * 2.80 / 3.50 = 80%
The new price, 2.80, is 80% of the original price. Since 100% - 80% is 20%, this means that the decrease of the price was 20%.
