New to Python

[w]

excuse my ignorance.  I cannot get this simple program to calculate
the formula.

The error message is unsupported operand type(s) str.  I know the data
that I'm reading from the file is a string but I cannot convert it to
a float or int.

The code a sample form the data file are below.

# Calculate Gross Profit from SIL File

import os.path

in_file = open( "s3cga1.new", "r")

while 1:
     data = in_file.readline()
     if not data:
	    break

     if data[0:1] == "(":

	nCost = data[ 99:106]
	nPack = data[ 44:48]

	nRetail = data[ 79:87]
	nQuan = data[ 87:90]
	nAllowance = data[ 147:154]

	nGp = ( ( nRetail / nQuan ) - ( ( nCost - nAllowance) / nPack ) ) / (
nRetail / nQuan )

	print nCost, nPack, nQuan, nRetail, nAllowance


EXAMPLE OF DATA FILE
(00007756725434,'*BREYER STRWBRY ICE ',0007,0006,' 56 OZ   ','000216  
      ',0004.95,001,2004260,0024.43000,0000000,00000.00,000,0000000,0000000,0004.00000,2004262,2004282),
(00007756725433,'*BREYER PEACH ICE CR',0007,0006,' 56 OZ   ','000224  
      ',0004.95,001,2004260,0024.43000,0000000,00000.00,000,0000000,0000000,0004.00000,2004262,2004282),