Which costs the last per ounce, a 20 ounce soda for $0.60, 68 ounce for $2.38, or 100 ounce for $3.32? Explain how you got your answer.

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  20 oz for $0.60

Step-by-step explanation:

You can determine the least-cost item by comparing costs at the same size. (This can be done using mental arithmetic.)

68 ounces is between 3 and 4 times the size of 20 ounces. If the costs were the same, 68 ounces would cost between 3·$60 = $1.80 and 4·$60 = $2.40. The cost of 68 ounces is closer to $2.40 than 68 is to 80, so we conclude the 20-ounce soda costs less.

The cost of 100 ounces at the 20-ounce price would be 5·$0.60 = $3.00, which is less than the cost of the 100-ounce soda. Again, the 20-ounce soda is cheaper.

The least cost per ounce is the 20-ounce soda.

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If you like, you can actually find the cost per ounce by dividing the cost by the ounces. Those results are shown in the attached table. The 20-ounce soda costs the least.