The proportions 2:3:5 have to be preserved.

So if we use $\simplify{{ratios[0]}*U}$ gallons of $x$ then we must use $\simplify{{ratios[1]}*U}$ gallons of $y$ and $\simplify{{ratios[2]}*U}$ gallons of $z$, to get $\var{ratiototal}U$ gallons of the preparation.

We would like $U$ to be as big as possible.

As we have $\var{u[0]}$ gallons of $x$, $2$ is not more than $12$, i.e. $U$ is not more than $6$.

As we have $\var{u[1]}$ gallons of $y$, $3$ is not more than $21$, i.e. $U$ is not more than $7$.

As we have $\var{u[2]}$ gallons of $z$, $5$ is not more than $30$, i.e. $U$ is not more than $6$.

So the maximum value of $U$ is $\var{lots}$ and we can make $\var{lots} \times \var{ratiototal} = \var{amount}$ gallons of the preparation.