839 - Math - Problem Solving

Question: X alone can do a piece of work in 15 days and Y alone can do it in 10 days. X and Y undertook to do it for Rs. 720. With the help of Z they finished it in 5 days. How much is paid to Z?

1. Rs. 360
2. Rs. 120
3. Rs. 240
4. Rs. 300