I am a newbie to Excel programming and have a problem I need to work out. The
publishing world has switched from 10 digit ISBNs to 13 digits by adding a
prefix 978 to an existing 9 digit string and calculating the last digit or
check digit. Our new SAP system is configured to only produce the 13 digit
now, but I still need the 10.

The formula for creating the 10 digit number is to strip off the prefix and
multiplying the next 9 digits individually using a declining weight, ie...

9781418918453 - 978 1*10, 4*9, 1*8, 8*7, 9*6...
Check Digit = (11 - Mod(Sum(1*10, 4*9, 1*8, 8*7, 9*6, 1*5, 8*4, 4*3, 5*2),11))
if Check Digit = 10 then Check Digit = "X"

I started by creating a bunch of variables using Mid() to pick out each
number in the string, but I keep getting type mismatch error. I am also
running my ISBNs in column A so I need to loop through.

Any help is greatly appreciated?