Question 757130
The description seems to deal with "masses". 

Let m= ounces of the 10% solution to use.


{{{(10m+20*4)/(m+4)=15}}}
And you know from the numbers given that m must be exactly 4 ounces, because you would want equal amounts of the 10% and the 20% to achieve the 15%, exactly in the middle of the two available solutions.


That above is the equation.  Solve for m.

{{{10m+20*4=15(m+4)}}}
{{{2m+16=3(m+4)}}}
{{{2m+16=3m+12}}}
{{{16=m+12}}}
{{{16-12=m}}}
{{{m=4}}}