Question 708615
If you want filling in with values, then for a of the 20% acid and b of the 35% acid, you can make these:

{{{(20*a+35*b)/3=25}}}  and  {{{a+b=3}}}


From the rational equation, multiply left and right by 3, and also divide both sides by 5 (seeing that all terms include a multiple of 5):
4a+7b=3*5,
{{{4a+7b=15}}}


We have this adjusted system to solve for a and b:
{{{4a+7b=15}}}
{{{a+b=3}}}


From that, you have a choice of methods.  Substitution, elimination, or simple matrix operations if you wish to.

Trying substitution, {{{a+b=3}}} gives us {{{b=3-a}}};
Put into {{{4a+7(3-a)=15}}}, and.....