Question 760343
Assign variables to all quantities.
L = 11%, for 11% salt
H = 18%, for 18% salt
T = 16%, the target value
M = 20 Liters, the target amount of solution
u = unknown volume of the 11% solution
v = unknown volume of the 18% solution


Form the equations for the situation.
Resulting salt concentration is (amount of pure salt)/(amount of salt solution result).
Sum of solutions used to prepare the target solution will be 20 liters.


{{{(Lu+Hv)/M=T}}} and {{{u+v=M}}}
Solve this system for u and v.


Here is a start:
{{{Lu+Hv=TM}}}
and from the volume sum equation, {{{v=M-u}}};
substitute for v,
{{{Lu+H(M-u)=TM}}}
{{{Lu+HM-Hu=TM}}}
{{{Lu-Hu=TM-HM}}}
{{{u(L-H)=M(T-H)}}}
{{{highlight(u=M(T-H)/(L-H))}}} a formula now for u,...
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