https://preparationpoint.info/mathematics-mcqs/p-can-lay-railway-track-between-two-stations-in-16-days-q-can-do-the-same-job-in-12-days-with-the-help-of-r-they-completes-the-job-in-4-days-how-much-days-does-it-take-for-r-alone-to-complete-the/