Class 5A can complete a project in 15 days while class 5B needs twice that amount of time. To complete this project before the summer festival, both class 5A and class 5B need to work together. Due to other tasks, class 5A has to leave for 3 days while class 5B has to leave for 6 days. Knowing that both classes do not leave at the same time, and they do work together on some days, what is the total number of days taken to complete this project? ​