In order for us to know the answer, we just have to know what percent is 18 from 25, given that they sold 25 t shirts last week and only 18 t-shirts this week. So to know this, we simply divide 18 by 25 and we get 0.72 multiply this by 100 and it is 72%. To know the percentage of decrease, simply subtract 72% from 100% and we get 28%. Therefore, the percentage of decrease is 28%. Hope this answer helps.