The hexadecimal system.
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16 16
Hexadecimal
No. For any base, n, you use the digits 0 to (n-1) for all numbers within that system. If, as in the case of the hexadecimal system (base 16), 0-9 are not sufficient to take you to n-1, you "borrow" some letters of the alphabet. In this case A ,B, C, D, E and F.
The number of digits in a valid credit card can change depending upon the type of card. Mastercard and VISA use a 16 digit system, while others including American Express use a 15 digit system.
A quadrillion has 16 digits.