Description for Correct answer:
Pipe A is opened for 8min.

Therefore, it will fill \( \Large \frac{8}{20} \)th of the tank. (That is, \( \Large \frac{2}{5} \) of the tank)

Remaining \( \Large \frac{3}{5} \)th of the tank has to be filled by A and B together.

A and B can together fill \( \Large \left(\frac{1}{20}+\frac{1}{30}\right)=\frac{1}{12} \)th of the tank in a minute.

Remaining tank will get filled in \( \Large \frac{3}{5} \times 12=\frac{36}{5} \) = 7.2 min

Therefore, total tank will be filled in 8 + 7.2 = 15.2 min