In a batch absorption process, $5\:g$ of fresh absorbent is used to treat $1$ liter of an aqueous phenol solution. The initial phenol concentration is $100\:mg$/liter. The equilibrium relation is given by

$$q^{*}=1.3C$$

where $q^{*}$ is the amount of phenol absorbed in $mg$ of phenol per gram of absorbent; and $C$ is the concentration of phenol in $mg$/liter in the aqueous solution.

When equilibrium is attained between the absorbent and the solution, the concentration of phenol in the solution, rounded to $1$ decimal place, is _____________ $mg$/liter.