E. coli is cultivated in a chemostat operated at a dilution rate of $0.2 h^{-1}$. The values of biomass yield due to oxygen consumption and the steady state biomass concentration are $0.2 g \cdot g^{-1}$ and $10 g \cdot L^{-1}$, respectively. The oxygen transfer rate
(in $g \cdot L^{-1} \cdot h^{-1}$) is ______________.