Question 159214
Let x=amount of soybean meal needed
Then 280-x=amount of cornmeal needed

Now we know that the amount of pure protein in the soybean meal(0.14x) plus the amount of pure protein in the cornmeal(0.07(280-x)) has to equal the amount of pure protein in the final mixture(0.12*280).  So, our equation to solve is:
0.14x+0.07(280-x)=0.12*280  get rid of parens and simplify
0.14x+19.6-0.07x=33.6  subtract 19.6 from each side
0.14x-0.07x+19.9-19.6=33.6-19.6  collect like terms
0.07x=14  divide each side by 0.07
x=200 lb-----------------------------------amount of soybean meal needed

280-x=280-200=80 lb----------------------amount of cornmeal needed

CK
200*0.14+80*0.07=280*0.12
28+5.6=33.6
33.6=33.6

Hope this helps---ptaylor