Question 1055238
Three-part mixture but you can turn this into something that looks like a two-part mixture problem.

<pre>
VOLUME     CONCENTRATION
x            5%
y           10%
z           15%

{{{z/x=4}}}
</pre>
Already an equation is identified.


{{{system((5x+10y+15z)/30=12,x+y+z=30)}}}
and again, you know one already found equation, useful as  {{{z=4x}}}.


{{{system((5x+10y+15(4x))/30=12,x+y+4x=30)}}}


{{{system(65x+10y=360,5x+y=30)}}}


{{{system(12x+2y=72,5x+y=30)}}}
If you think carefully for a few seconds, you can continue this partly through Elimination Method.


(  x=6  )