13 x - 7 / 10 x ^ 2 - 11 x + 3
2 answers:
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so hmm notice, we use the decimal format for the percent, namely 15% is just 15/100 and 5% is just 5/100 and so on
whatever the amounts of "x" and "y" are, they must add up to 100 grams
thus x + y = 100
and whatever the concentrated amounts are, they'll add up to (100)(0.15)
thus
solve for "x", to see how much 5% alloy will be needed
what about "y"? well y = 100 -x
X * (x-2) = x*x - 2*x = 
- 2x