Exact amount payable for arrear deamand

I received an arrear demand of 20K from CPC for AY-2008. The demand further mentions that interest has been calculated till 20-03-2011. Before paying up the arrear amount, I need to know the exact amount to be paid up since interest has been calculated till the month of March, 2011. Is there any "demand order" corressponding to the arrear demand notice from IT? If yes, then how do I collect the demand order?