Help please!! I'm so confused

2-digit numbers less than 91 which are 1 less than a multiple of 10

Adepartment store purchases screen-printed t-shirts at a cost of $5 per shirt. they mark up the price 150% (making the selling price 250% of the store's purchase price) and put them on the sales floor. every month that a t-shirt doesn't sell, the store reduces the selling price by 25%.

Which property was used to simplify the expression? 3c+9 +4c=3c+4c+9