SOLUTION: how much 20% acid must be mixed with 5% acid to make 30mL of 12% acid?

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Question 769271: how much 20% acid must be mixed with 5% acid to make 30mL of 12% acid?
Answer by josgarithmetic(39617) About Me  (Show Source):
You can put this solution on YOUR website!
ASSIGN VARIABLES TO ALL QUANTITIES
L, the lower concentration of available material
H, the higher concentration of available material
T, the target concentration of resulting mixture
x, the amount of lower concentrated material to use, unknown
y, the amount of higher concentrated materaial to use, unknown
M, amount of the resulting mixture

FORM EQUATIONS
highlight%28%28Lx%2ByH%29%2FM=T%29
highlight%28x%2By=M%29

Most of the symbolic solution process:
%28Lx%2ByH%29%2FM=T
Lx%2ByH=TM
Lx%2B%28M-x%29H=TM
Lx%2BHM-Hx=TM
%28L-H%29x%2BHM=TM
%28L-H%29x=TM-HM
%28L-H%29x=%28T-H%29M
x=M%28T-H%29%2F%28L-H%29, and since the two differences here would both be negative,
we can multiply by (-1)/(-1),
x=M%28H-T%29%2F%28H-L%29

Do what you need to find y.
Substitute given values for finding values of x and y.