Answer:
Ruff should use 109 ounces of grain only in order to minimize cost.
Explanation:
For Chicken
Protein in Chicken = 10 grams of protein per ounce
Fat in Chicken = 5 grams of fat per ounce
Number of chicken ounce needed for protein = 218/10 = 21.8
Number of chicken ounce needed for fat = 168/5 = 33.6
Chicken cost = 10¢ per ounce
Cost of chicken required for protein = 21.8 * 10¢ = 218¢
Cost of chicken required for fat = 33.6 * 10¢ = 336¢
For Grain
Protein in Grain = 2 grams of protein per ounce
Fat in Grain = 2 grams of fat per ounce
Number of grain ounce needed for protein = 218/2 = 109
Number of grain ounce needed for fat = 168/2 = 84
Chicken cost = 1¢ per ounce
Cost of grain required for protein = 109 * 1¢ = 109¢
Cost of grain required for fat = 84 * 1¢ = 84¢
Conclusion
In order to minimize cost, Ruff should use 109 ounces of grain only in each bag of dog food to obtain at least 218 grams of protein and even more than 168 grams of fat.