You are buying potato chips at the grocery store. you have a manufacturer’s coupon for $1.00 off and the store is offering 20% off if you buy 2 bags. how do you compute your final cost?

You take your cost of the chips (Lets say it is 2.50 a bag so $5 for two bags).
20% off means it will be at 80% of the full price so you multiply 

5 x 0.8 = 4

Now if you add in  the discount of your $1 coupon, you have 4 - 1 = 3 so your final answer would be $3. 

(Keep in mind, I was given no cost for the chips so I made one up. This is just to explain the procedure.)

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