0-9: Same as decimal.

A: Decimal 10

B: decimal 1 1

C: decimal 12

D: decimal 13

E: decimal 14

F: decimal 15

That is to say, the maximum hexadecimal number of a number is f, decimal means 16, hexadecimal means 10, and so on.

Hexadecimal 0 1FF means that the first digit is f, the second digit is also f, and the third digit is 1, that's all.

Hexadecimal 0 1FF = decimal1* 256+16+15 = 51.