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Notes on Unit Rate Comparisons

Suppose you are buying large quantities of an item such as hamburger rolls.  Costco offers 250 rolls for $5.49 and Waldbaums offers 42 rolls for $1.29.  Which is the better buy?

In order to solve this problem you must find the unit rate of each store, since you can not compare the cost of different amounts.  In other words how much is one roll at Waldbaums and how much is one roll at Costco?

To find the unit rate for Costco set up a proportion: 

Number of rolls 250 1
Cost of rolls 5.49 n

    Then cross multiply to solve:  250n = 5.49             $.02 per roll

 

To find the unit rate for Waldbaums set up a proportion: 

Number of rolls 42 1
Cost of rolls 1.29 n

    Then cross multiply to solve:  42n = .1.29           $.03 per roll

 

Now by comparing the unit rate for Costco of $.02 per roll and the unit rate for Waldbaums of $.03 per roll you can see that Costco is the better buy.  One cent might not seem like a lot, however if you had a store and needed 100,000 rolls per week that would be $1000 dollars for your business.