A reverse osmosis unit treats feed water (F) containing fluoride and its output consists of a permeate stream (P) and a reject stream (R). Let C${_F}$, C${_p}$, and C${_R}$ denote the fluoride concentrations in the feed, permeate, and reject streams, respectively. Under steady state conditions, the volumetric flow rate of the reject is 60% of the volumetric flow rate of the inlet stream, and C${_F}$ = 2 mg/L and C${_P}$ = 0.1mg/L.
The value of C${_R}$ in mg/L, up to one digit after the decimal point, is $\_\_\_\_\_\_\_$