Is the number totally obscured? Can you make out any of the digits? If so, try to enter what you 'think' is the number. If paypal won't take your card information because the security code is not entered correctly, or not entered at all, call your credit card issuer and ask how you can get your code, which is now illegible on the back of your card. If you can answer any number of security questions, they might give it to you, but there's also a chance they will either offer to mail it to you or issue you a new card with a new credit card number, or the same number and only a new three or four digit security number on the back. Unfortunately, no, you can't figure out the number mathematically. It's randomly assigned by a computer when your card is printed.
While I realize you need the info in your paypal account by tomorrow morning, if the credit card company offers to mail you a new card with a 'fresh' number on the back, this won't do, immediately; however, if you possibly can use another credit card where the number panel on the back is legible, this might be the next best thing, for the moment.
In the future, try to keep your credit cards in those little plastic slide-in pockets, in your wallet, so the numbers don't get rubbed off.
I hope you are successful!
If you have found this answer helpful, please click 'Accept' and leave positive feedback; if you need additional information, please click 'Reply'. Thanks for the opportunity to be of help.
Edited by Cher on 4/8/2010 at 3:57 AM EST