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The nutritional content per ounce of 3 foods is presented by the table on the link. If a meal consisting of the 3 foods allows exactly 1600 calories, 108 grams of protein, and 2900 milligrams of Vit C, how many ounces of each kind of food should be used?

The question is posted on the following link along with the answer, I would like to know what process is used.

Thank you