The decomposition of $N_2{O}_5$occurs as $2{\mathrm{N}}_2{\mathrm{O}}_5\to 4{\mathrm{NO}}_2+{\mathrm{O}}_2$and follows 1st order kinetics, hence 

Half life time $\left({\mathrm{t}}_{1/2}\right)$ for nth order reaction is given by, $t_{1/2}\propto {\left[a\right]}^{1-n}$

where n is the order of reaction and a is concentration of reactant.

As decomposition of $N_2{O}_5$ follows 1st order kinetic.

So, $\Rightarrow t_{1/2}\propto {\left[a\right]}^{1-1}\Rightarrow t_{1/2}\propto a°$