Description for Correct answer:
Let 15 men work for m days.

Work done in 1 days = \( \Large \frac{m}{20} \)

Remaining work = \( \Large \left(1 - \frac{m}{20}\right) \)

25 men's 1 days work = \( \Large \frac{1}{20} \)

1 man's 1 days work = \( \Large \frac{1}{20} \times \frac{1}{25}=\frac{1}{500} \)

10 men's 1 days work= \( \Large \frac{1}{500} \times 10 = \frac{1}{50} \)

10 men's \( \Large \frac{75}{2} \) days work

=\( \Large \frac{1}{50} \times \frac{75}{2}=\frac{75}{100}=\frac{3}{4} \)

Therefore, \( \Large \left(1 - \frac{m}{20}\right)=\frac{3}{4} => \frac{m}{20}=\frac{1}{4} \)

=> \( \Large m = \frac{1}{4} \times 20 \)

Clearly, 15 men leave after 5 days.