Question 444253
Here's a sample problem that I solved:
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Soybean is 18% protein
Cornmeal is 9% protein
How many lbs of each should be mixed together to get 360-lb mixture that is 12% protein?
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Let a = pounds of soybean needed
Let b = pounds of cornmeal needed
given:
 .18a = protein in soybeans
 .09b = protein in cornmeal
(1) a + b = 360
(2) ( .18a + .09b ) / 360 = .12 
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(2) .18a + .09b = .12*360
(2) .18a + .09b = 43.2
(2) 18a + 9b = 4320 
Multiply both sides of (1) by  9 and
subtract (1) from (2)
(2) 18a + 9b = 4320 
(1)  -9a  - 9b = -3240
 9a = 1080
a = 120
and
(1) a + b = 360
b = 360 - 120
b = 240
120 pounds of soybeans are needed
240 pounds of cornmeal are needed
check answer:
(2)  ( .18a + .09b ) / 360 = .12 
(2) .18*120 + .09*240 = .12*360
(2) 21.6 + 21.6 = 43.2 
(2) 43.2 = 43.2
OK