A stream flowing at 15 $\text{m}^3/\text{s}$ has a tributary feeding into it with a flow rate of 7 $\text{m}^3/\text{s}$. The concentrations of chloride at the upstream of the junction and that of the tributary are 30 mg/L, and 50 mg/L respectively. Treating chloride as conservative substance and assuming complete mixing of two streams, the concentration of chloride in mg/L at the downstream is ______.