$$CH_4+O_2 \rightarrow HCHO+H_2O \\ CH_4 +2O_2 \rightarrow CO_2 +2H_2O$$ Methane and oxygen are fed to the reactor. The product gases leaving the reactor include methane, oxygen, formaldehyde, carbon dioxide and water vapor.
$60 \text{ mol s}^{-1}$ of methane enters the reactor. The molar flowrate (in $\text{mol s}^{-1})$ of $CH_4, O_2$ and $CO_2$ leaving the reactor are $26, 2$, and $4$ respectively. The molar flowrate of oxygen entering the reactor is _______ $\text{mol s}^{-1}$