Which store is the better buy if 20 ounces of pecans cost 7.80 at Food Depot whilst 32.5 ounce bag of pecans costs 7.15 at Food Mart?

You need to compare the rates for each store. Take dollars per ounce (dollars divided by ounces). The lower number is less expensive, and the better deal. You could also take ounces per dollar (ounces divided by price), which will tell you how many ounces that a dollar will buy. In that case, the higher number is better, since you can get more ounces for the same amount of money. Either method will get the right answer for you.