Compound Interest Calculation problem (sos)

SET @MMCOMPCNT = @ACOMPCOUNT 
	SET @CCCMTH    = @CMONTH  
	SET @CCCYR     = @CYEAR 

											
	--COMPUTING THE DEDUCTION BEGIN
	--COMPUTING THE DEDUCTION BEGIN
	SET @MMRATE   =  @BRATES/(100*12) 
	SET @ZRATE    =  1+@MMRATE
	SET @MTERM    =  @MMCOMPCNT
	SET @MMTEMP   =  POWER(@ZRATE,@MTERM) 
	SET @MMDEDUCT = (@AOUTST*@MMRATE)/(1-(1/@MMTEMP))	
	--COMPUTING THE DEDUCTION END
	--COMPUTING THE DEDUCTION END										

case "MONTHLY REPAYMENT":

                               double xxprincipal = Convert.ToDouble(txt_Principal.Text);
                               double xxinterest   = Convert.ToDouble(txt_Interest.Text);

                               double xxmyear = Convert.ToInt32(txt_Term_Year.Text);
                               double xxmonth = Convert.ToInt32(txt_Term_Month.Text);

                               double mmrate = xxinterest/(12*100);
                               double zrate = 1 +mmrate;

                               double mterm     = 12 * xxmyear + xxmonth;

                               double mmtemp     = Math.Pow(zrate, mterm);

                               double mrepayment=(xxprincipal*mmrate)/(1-(1/mmtemp));

                               txt_Monthly.Text = mrepayment.ToString("N");