A radioactive nucleus can decay in two ways…

Step 1. The half-life. $\lambda = \lambda_1 + \lambda_2 = 0.693\left(\dfrac{1}{20\ \text{min}} + \dfrac{1}{30\ \text{min}}\right) = \dfrac{0.693}{12\ \text{min}}$. So the half-life is $T = 12$ min. In a short way, $\dfrac{1}{T} = \dfrac{1}{T_1} + \dfrac{1}{T_2}$.

Step 2. The share. $\dfrac{\lambda_1}{\lambda} = \dfrac{1/20}{1/12} = \dfrac{12}{20} = 0.60$, which is $60\ \%$.

Step 3. After $36$ min. That is $\dfrac{36}{12} = 3$ half-lives, so $\left(\dfrac{1}{2}\right)^3 = 0.125$, or $12.5\ \%$.