A, B and C take a contract to do a piece of work. A can do it alone in 10 days. B can do it in 20 days and C can do it in 40 days. They work alternately. On the 1st day, A will do it alone, on the 2nd day, B will do it and on the 3rd day, C will do it alone and so on till the work is completed. Find the time in which work will be completed.