For the first order reaction : ${\mathrm{H}}_2{\mathrm{O}}_2\left(aq\right)⟶{\mathrm{H}}_2\mathrm{O}\left(l\right)+\frac{1}{2}{\mathrm{O}}_2\left(g\right)$

the half life of reaction is 30 min. If the volume of ${\mathrm{O}}_2$(g) collected at a certain pressure and temperature is 100 ml after a long time from the start of reaction, then what was the volume of ${\mathrm{O}}_2$(g) collected at the same pressure and temperature after 60 min from the start of reaction?