A certain number of men can finish a piece of work in 10 days If however there were 10 men less it will take 10 days more for the work to be finished How many men were there originally?

If a "certain number of men" less 10 takes 20 days (twice as long) to finish the piece of work, then the "certain number of men" who can finish the work in 10 days would 20. If it takes 10 men less twice as long to do the work, then twice 10 men (20) can finish the job in 10 days.