80 men can complete a work in 30 days. They start work together and after every 10 days 10 men left the work. In how much time work will be completed?

Total work = 80 × 30 = 2400 man-days. Work completed in first 10 days with 80 men = 800 man-days. Work in next 10 days (11-20) with 70 men = 700 man-days. Work in next 10 days (21-30) with 60 men = 600 man-days. After 30 days, total work done = 2100 man-days. Remaining work = 300 man-days with 50 men, which takes 300/50 = 6 days. Total time = 30 + 6 = 36 days.