Dear friend,
Your query compels me to doubt whether your opening statement is correct.
Firstly, if you can say that your gross salary is Rs.15500-00 ( should be per mensem though not mentioned so ), it should certainly comprise of different components in which case there is no necessity to ask about basic and if it is a lump sum, then it should be consolidated salary warranting your question about the basic salary.
Secondly, in the absence of other benefits you get which can be computed in terms of money and the amounts of employer's contributions to EPF,ESIC,Gratuity Fund etc., one can not arrive at the actual amount of your CTC.