Explanation:

- Initially, 20 men can finish the work in 50 days.

- Hence, total work units = 20 men * 50 days = 1000 man-days.

- For the first 10 days, these 20 men complete 20 men * 10 days = 200 man-days of work.

- Remaining work is 1000 - 200 = 800 man-days.

- After 10 days, 30 more men join. So, total men now = 20 + 30 = 50 men.

- 50 men complete the remaining 800 man-days of work.

- Days needed to complete 800 man-days = 800 man-days / 50 men = 16 days.

- Total days = 10 days + 16 days = 26 days.

- Originally, the project was planned for 50 days.

- Days saved = 50 - 26 = 24 days.

- Option:3 - 24 days is the correct answer.