Question 750796
(amount of acid)/(amount of mixture)=fraction as the acid or the concentration


Some parts are variable.


Let u = liters volume to add of the 7% acid.
Let v = liters of 27% acid.


Amount of acid = u*7%+v*27%
{{{(7u+27v)/780=13}}}  and {{{u+v=780}}}
Although one of these is rational equation, they both are linear equations in two variables.
Just solve the system for u and v.