I am thinking you are referring to the mark-up price of an item. The "mark-up" price of an items is the amount a store adds to its cost of an item in order to make a profit when it is sold. For example, if an item costs Wal-Mart $10 and they add $1 to the cost and sell it to customers for $11 dollars, they have a $1 "mark-up" or a 10% mark-up.
These types of problems are all about setting up ratios. You will be given three of the four slots (you always use 100) and will need to find the fourth by cross multiplying.
markup amount / cost of item to store = percentage/100
I always tell my students to use this formula
____is / of ______ = _______%/100
So, using our first example:
1 is/of 10 = _____%/100
Cross multiplying gives us 1*100 = 10x or 100 = 10x or 10 = x. So, the store's mark-up is 10%.