Vishal,
Please read the FAQ on Salary, it is also covering HRA part with example.
Now as per income tax Act the HRA Exemption Available is least of the following -
1. HRA received
2. 40% of Basic + DA
3. Rent paid in excess of 10% of basic + DA.
So exemption for HRA is depends on HRA recieved, rent paid and basic+DA.