Operational research self study

i am CA final student. Initially i found OR is easy so i missed all the lectures of OR bcz of faculty was not good. I thought i can do self study of OR bcz from initial chapter i found it easy. But now problem is that i have started self study from practise manual but except Assignment and transportation (not all the sums i understand) i didnt get all other chapters. My frind told me that OR is scoring and help to pass costing subject but at the same time if u have not attended coaching and consepts not cleared then it is too tough also and can not clear gud marks in that scoring sums after 2-3 attempts also u will confuse how to solve all this.. And same happening with me..  Practise manual is not helping me for few chapters of OR for self study. So let me know shud i go for crash course of OR now which is available at the cost of 5000 to 6000 Rs. or is there any books or notes which is easy self explaining and from which i can do self study of OR chapters..