https://preparationpoint.info/mathematics-mcqs/basic-maths-mcqs/a-and-b-can-do-a-piece-of-work-in-21-and-24-days-respectively-they-start-the-work-together-and-after-some-days-a-leaves-and-b-completes-the-rest-of-the-task-in-9-days-after-how-many-days-did-a-leave/