Question 703984
Notice that 15 is exactly in the middle of 9 and 21. EXACTLY.  Maybe half of 540 liters of each solution?


x and y are liters volume of the 9% and the 21% respectively, to use.
x+y=540.


Account for the percntages,
{{{(x*0.09+y*0.21)/540=0.15}}}
{{{(x*0.09+y*0.21)=0.15*540}}}
{{{9*x+21*y=15*540}}}
Substitute x=540-y into this equation,
{{{9*(540-y)+21*y=15*540}}}
Continue to find y...
{{{12y=15*540-9*540}}}
{{{y=540*(6/12)}}}
{{{y=270}}} liters