Clarification on service tax on pay and park facilities

Dear All, i have a query regading the application of service tax.please guide and advice A is Owner of Mall, leased ou parking lot to B for monthly rent /consideration/fees of Amount Rs X. B being a contractor /collectng agent who retains the revenue collected and pay monthly rent of amount X total amount collected is Y Rent paid to A is X plus service tax Expenses(salaries and others) is Z now my question is how much service tax is B liable to pay to govt