I'm trying to use strings to do math with very large numbers using strings, and without external libraries.

I have tried looking online with no success, and I need functions for addition, subtraction, multiplication, and division (if possible, and limited to a specified number of decimal places.)

example: add 9,900,000,000
and 100,000,020
should be 10,000,000,020.

EDIT: Im sorry I diddn't be specific enough, but I can only use Strings. no Long, bigInt, anything.
just the basic string and if nessecary, int32.

This is NOT a homework question!