What amount to be calculatatio for service tax?

I am requesting you to please help me to calculate service tax @ 15%. ex: i was recently appointed as an accountant for an educational instistute. They collect 95000/- fee from student. out of 95000/-, rs.12000/- deduct for library fund. Now my doubt is service tax 15% calculation should i calculate either on 95000/- or 83000/-. Please help me to calculate st