SALE PRICE DISCOUNT Percent of change is % of discout and % of mark up (decrease and increase) 1st step: AMOUNT OF CHANGE: 36-21=15.(i think it goes like this).. 36+21=57 2nd step:PROPORTION: 15/36 x/100 (cross multiply 15 with100 then divide that number by 36)Same thing except put in 57 instead of 15