Description for Correct answer:
Part of the tank filled in 1 min by A, B and C.

= \( \Large \frac{1}{20}+\frac{1}{15}+\frac{1}{12} \)

= \( \Large \frac{3+4+5}{60} = \frac{12}{60} = \frac{1}{5} \)

Therefore, Time taken by A, B and C to fill the tank = 5 min.

Therefore, Time taken by A, B and C to fill 40% of the tank

= 40% of 5 = \( \Large \frac{40}{100} \times 5 \)