For a First-Order Reaction, the half-life ($t_{1/2}$) is constant.

• Let $[A{]}_0$ be the initial concentration.
• After 1st half-life ($20\text{ min}$), $[A]=50\%$ of $[A{]}_0$.
• After 2nd half-life (another $20\text{ min}$), $[A]=50\%$ of $50\%=25\%$ of $[A{]}_0$.
• 75% completion means $25\%$ reactant is remaining.
• Total time $=1$st half-life $+2$nd half-life $=20+20=40\text{ minutes}$. Alternatively, for first order, $t_{75\%}=2\times t_{50\%}$.