What is the half life of a first order reaction if time required to decrease concentration of reactant from 0.8 M to 0.2 M is 12 hour?

For the reaction $2 \mathrm{~N}_2 \mathrm{O}_{5(\mathrm{~g})} \longrightarrow 4 \mathrm{NO}_{2(\mathrm{~g})}+\mathrm{O}_{2(\mathrm{~g})}$ rate and rate constant are $1.02 \times 10^{-4} \mathrm{~mol} \mathrm{~L}^{-1} \mathrm{~s}^{-1}$ and $3.4 \times 10^{-5} \mathrm{~s}^{-1}$. What is the conc. of $\mathrm{N}_2 \mathrm{O}_5$ ?

What is the order of following reaction

$$2 \mathrm{H}_2 \mathrm{O}_{2(\mathrm{~g})} \longrightarrow 2 \mathrm{H}_2 \mathrm{O}_{(\mathrm{l})}+\mathrm{O}_{2(\mathrm{~g})}$$

Half life of a first order reaction is 1 hour. What fraction of it will remain after 3 hour?