If a first-order reaction completes to $\mathrm{z}\%$ in $20$ minutes and to $19\%$ in $40$ minutes, then the value of $\mathrm{z}$ is

The rate of a first-order reaction is $0.04 \mathrm{mol} {\mathrm{L}}^{-1}{\mathrm{s}}^{-1}$ at $10$ minutes and $0.03 \mathrm{mol} {\mathrm{L}}^{-1}{\mathrm{s}}^{-1}$ at  $20$ minutes after initiation of the reaction. The half-life of the reaction is