Sankar and Vishnu work alternately in a tailoring shop on an order of stitching 21 suits. Sankar starts the work, stitches 3 suits in two days, and takes a break, during which Vishnu stitches 3 suits in three days, and then takes a break. This pattern is repeated till all the 21 suits are stitched. How many days did it take the duo to complete the work?