The number of days to finish a work by A is double the number of days to finish the work by B. The number of days to finish the work by C is half the number of days to finish the work by B. A can finish the work in 12 days. If B works on the 1st day, A works on the 2nd day and C works on the 3rd day, again B works on the 4th day and so on, then the work is finished in ______ days.