Question 713345: A wild bird seed mix is made by combining 150 lb of millet seed costing $0.40 per pound with sunflower seeds costing $1.30 per pound. How many pounds of sunflower seeds are needed to make a mixture that costs $0.70 per pound?
lb
Answer by josgarithmetic(39617)   (Show Source):
You can put this solution on YOUR website! Solve in general first, especially if you have other problems like that one.
ASSIGN VARIABLES TO ALL QUANTITIES
L, the lower concentration of available material, $0.40/pound
H, the higher concentration of available material, $1.30/pound
T, the target concentration of resulting mixture, $0.70/pound
x, the amount of lower concentrated material to use, 150 pounds
y, the amount of higher concentrated material to use, UNKNOWN
That can be partly changed by multiplying both sides by x+y to get  .
 , and since the differences in parentheses would both be negative, we can multiply left side and right side members by -1 to get,
In case x were known but y were the unknown, our solution would be the formula for y,
 , THIS is what we want in this bird seed mix problem.
NOW substitute the known values from given in the problem description.
 , pounds of $1.30/pound sunflower seeds to add.
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