person A was given a job to completed in 20days. But after working for 5 days, he was asked to finish the job in total of 15 days. For this, he included person B to join him from the 6th day onwards such that they would be able to finish the job exactly at the end of 15 days. But after the 10th day, they were asked to finish the job in total 13 days. So, A included another person C who would join them from the 11th day onwards such that the work would be completed exactly at the end of 13 days. Had all three been working together from the beginning, how many days would they have taken to finish the job?