Question 145889
An animal feed is made from three ingredients: corn, soybeans, and cottonseed. One unit of each ingredient provides units of protein, fat, and fiber, as shown below: 
Each unit of corn provides .25 units of protein, .4 units of fat, and .3 units of fiber.
Each unit of soybeans provides .4 units of protein, .2 units of fat, and .2 units of fiber.
Each unit of cottonseed provides .2 units of protein, .3 units of fat, and .1 units of fiber. 
How many units of each ingredient should be used to make a feed that contains 22 units of protein, 28 units of fat, and 18 units of fiber?
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Let amt. of corn be "r"; amt. of soybeans be "s", amt of cottoseed be "c".
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EQUATIONS:
Protein: 0.25r + 0.4s + 0.2c  = 22
Fat....: 0.4r + 0.2s + 0.3c = 28
Fiber..: 0.3c + 0.2s + 0.1c = 18
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Solve the system of equations to get:
r = 40
s = 15
c = 30
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Cheers,
Stan H.