Description for Correct answer:
Part of the tank filled by both taps in 5 min

=\( \Large 5 \times \left(\frac{1}{20}+\frac{1}{60}\right)

= \frac{5 \times \left(6+2\right) }{120}=\frac{8}{24}=\frac{1}{3} \)

Therefore, Remaining part = \( \Large \left(1-\frac{1}{3}\right)=\frac{2}{3} \)

Therefore, \( \Large \frac{1}{60} \) part is now filled in 1 min.

Therefore, \( \Large \frac{2}{3} \) part is now filled in \( \Large 60 \times \frac{2}{3} \) = 40 mm