Credit Card Numbers: What Do They Mean?

You'll find your credit card number either on the front or on the back of the card. Credit cards usually have 15 or 16 ...
Scientific American

The Math Trick Hidden in Your Credit Card Number

You’re at the checkout screen after an online shopping spree, ready to enter your credit card number. You type it in and instantly see a red error message ...

You can use most credit cards on PayPal – here's how to add a card and link it with your account to make purchases

You can use credit or debit cards on PayPal by adding one to your account through the "Banks and Cards" or "Wallet" menu.