To solve a sum dealing with number of men and number of days taken to complete a work use the logic: 1 person takes 1 day to complete 1 unit of work. If 20 men take 20 days to complete a work, How many days it will take 40 men take? Assume 1 person takes 1 day to complete 1 unit of work. 20 men complete 20 units in 1 day. So in 20 days, 400 units is completed. Hence 40 men will complete 40 units in 1 day and 40 men will complete 400 units in 10 days. Answer is 10 days. 
