Question 964428
The second, concentration equation:
{{{(10x+30y)/10=25}}}
should become
{{{10x+30y=250}}}------this will be easier to work with.


You now have the system
{{{x+y=10,10x+30y=250}}}


Simplifiable to 
{{{system(x+y=10,x+3y=25)}}}


Simply subtract the first equation from the second equation to eliminate x.
{{{2y=15}}}
{{{y=15/2}}}
{{{highlight(y=7.5)}}}; and from the necessary 10 liters of the 10%, you have {{{highlight(x=2.5)}}}.