a
$\mathrm{P}_{1}=\frac{\mathrm{K}_{1} \mathrm{A}(100-0)}{\mathrm{L}} \Rightarrow Q=\mathrm{P}_{1} \mathrm{t}_{1}$
$\mathrm{P}_{2}=\frac{\mathrm{K}_{2} \mathrm{A}(100-0)}{\mathrm{L}} \Rightarrow \mathrm{Q}=\mathrm{P}_{2} \mathrm{t}_{2}$
$\mathrm{P}_{3}=\frac{\mathrm{P}_{1} \mathrm{P}_{2}}{\mathrm{P}_{1}+\mathrm{P}_{2}}=\frac{\frac{\mathrm{Q}}{\mathrm{t}_{1}} \times \frac{\mathrm{Q}}{\mathrm{t}_{2}}}{\frac{\mathrm{Q}}{\mathrm{t}_{1}}+\frac{\mathrm{Q}}{\mathrm{t}_{2}}}=\frac{\mathrm{Q}}{\mathrm{t}_{1}+\mathrm{t}_{2}}$
$\Rightarrow Q=P_{3}\left(t_{1}+t_{2}\right)$
$\Rightarrow \mathrm{t}_{10}=\mathrm{t}_{1}+\mathrm{t}_{2}=100 \mathrm{minutes}$
$\mathrm{P}_{2}=\mathrm{P}_{1}+\mathrm{P}_{2}=\frac{\mathrm{Q}}{\mathrm{t}_{1}}+\frac{\mathrm{Q}}{\mathrm{t}_{2}}=\frac{\mathrm{Q}\left(\mathrm{t}_{1}+\mathrm{t}_{2}\right)}{\mathrm{t}_{1} \mathrm{t}_{2}}$
$\Rightarrow Q=P_{4}\left(\frac{t_{1} t_{2}}{t_{1}+t_{2}}\right)$
$\Rightarrow \mathrm{t}_{20}=\frac{\mathrm{t}_{1} \mathrm{t}_{2}}{\mathrm{t}_{1}+\mathrm{t}_{2}}=\frac{20 \times 80}{100}=16 \mathrm{min}$