A and B undertake a contract of a task for Rs. 10,800. A and B can complete the task in 45 days and 60 days, respectively. However, to finish the work early, they take C's help and complete the entire work in 20 days. What is the value (in Rs.) of (a - b + 2c), where a, b and c are the shares of A, B and C, respectively, for their contribution to complete the task?