For the first order reaction ${\mathrm{C}}_2{\mathrm{H}}_4\mathrm{O}\left(\mathrm{g}\right)\to {\mathrm{CH}}_4\left(\mathrm{g}\right)+\mathrm{CO}\left(\mathrm{g}\right)$,the initial pressure of ${\mathrm{C}}_2{\mathrm{H}}_4\mathrm{O}\left(\mathrm{g}\right)\text{ is }80$ torr and total pressure at the end of 20 minutes is 120 torr. The time needed for 75% decomposition of ${\mathrm{C}}_2{\mathrm{H}}_4\mathrm{O}$would be: 

Let $\mathrm{x}\text{ torr of }{\mathrm{C}}_2{\mathrm{H}}_4\mathrm{O}$ decompose after 20 min. Then, $80-\mathrm{x}+2\mathrm{x}=120;\mathrm{x}=40\text{ torr }=50\mathrm{\%}$ of initial pressure.
Hence ${\mathrm{t}}_{1/2}=20\min .\text{ For }75\mathrm{\%}$reaction, fraction left 
$=\frac{25}{100}=\frac{1}{4}={\left(\frac{1}{2}\right)}^2$
No. of half lives = 2. Time needed for 75% reaction., 2x20 =40 min