Question 1018007
<pre>
a   volume solution A   concentration 10%
b   volume solution B   conc,  15%
c   volume solution C   conc, 30%

a=3b

100 ml. of solution wanted, 15% concentration result.
</pre>

This is your starting system:   {{{system(a+b+c=100,10a+15b+30c=100*15,a=3b)}}}


Immediately simplify the concentration equation.
{{{2a+3b+6c=20*15}}}
{{{2a+3b+6c=300}}}


And substitute for a in the volume and simplified concentration equations.
{{{system(3b+b+c=100,2*3b+3b+6c=300)}}}
{{{system(4b+c=100,9b+6c=300)}}}
and one more simplification...
{{{highlight(system(4b+c=100,3b+2c=100))}}}


Not a big problem from here.  You finish.