Question 859008
You need the densities.
Percent Weight_______density g/ml
8____________________1.0376
12.2_________________est. 1.059
15___________________1.0723


(referenced http://www.handymath.com/cgi-bin/hcltble3.cgi?submit=Entry ).


The equations rely on being strictly in percents weight per weight.

Let u = VOLUME of 8% acid
Let v = volume of 15% acid


{{{(u*8*1.0376+v*15*1.0723)/(100*1.059)=12.2}}}, accounts for the percent concentration;
and
{{{u+v=100}}} should account well for the volumes.


Those two equations as a system will really be found to be two linear equations in variables u and v.