Description for Correct answer:
Part filled by 1st pipe = \( \Large \frac{1}{20} \)

Part filled by 2nd pipe in 1 min = \( \Large \frac{1}{24} \)

Part filled by all the pipes in 1 min = \( \Large \frac{1}{15} \)

Work done by the waste pipe in 1 min

= \( \Large \frac{1}{15}- \left(\frac{1}{20}+\frac{1}{24}\right)=\frac{1}{15}- \left(\frac{6+5}{120}\right) \)

= \( \Large \frac{1}{15}-\frac{11}{20} \)

= \( \Large \frac{8-11}{120}= \left(-\frac{3}{120}\right)= \left(-\frac{1}{40}\right) \) [-ve sign indicates emptying]

Now, volume of \( \frac{1}{40} \) part = 6 gallon

Therefore, Volume of whole tank = \( \Large 40 \times 6 \) = 240 gallon