Chemical Kinetics

In a first-order reaction, the initial concentration of $A$ is $0.5 \mathrm{~mol} \mathrm{~L}^{-1}$. After 20 minutes, the concentration reduces to $0.4 \mathrm{~mol} \mathrm{~L}^{-1}$. The rate constant is (in $\mathrm{min}^{-1}$ ).