Whats the question?
You'll need 15 bags to cover 15 yards
1 bag/ 3 feet
3 feet/ 1 yard
15 bags = 45 feet = 15 yards
The price after the sale is (18/22.5)*100 = 80% from the original price
so, the percentage of decrease in the price of the shirt is 100% - 80% = 20%
another way,
how much is decreased / original price *100
(22.5-18) / 22.5 *100 = 20%
I don't know if it is correct but here goes nothing 8*(d-3)