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