Question 1015387
If you mean, LITERS, then use an abbreviation that cannot be misunderstood, like either L or the word, "liters".


Make a formula to use starting if possible, with all symbols, because you will compute twice - you want a range of values for how much of the 30% acid to use.


T, the target resulting value from 15% to 18% acid
v, how much of the 30% acid to use
You could choose to use only those as the variables.


{{{(600*12+30*v)/(v+600)=T}}}
Solve for v.


{{{600*12+30v=T(v+600)}}}
{{{30v+600*12=Tv+600T}}}
{{{Tv+600T=30v+600*12}}}
{{{Tv-30v=600*12-600T}}}
{{{v(T-30)=600*12-600T}}}
{{{highlight(v=(600*12-600T)/(T-30))}}}


What is v for T=15?
What is v for T=18?
Those are the boundaries for v.