31 would be 0x1F. The hexadecimal number scheme has 16 symbols (since it has sixteen digits (base 16) instead of decimal (base 10)).
The decimal number sequence which everyone uses is: 0 1 2 3 4 5 6 7 8 9.
The hexadecimal number sequence is: 0 1 2 3 4 5 6 7 8 9 A B C D E F.
In hexadecimal: 0 through 9 map to their decimal counterparts, but since there are 6 more digits in base 16 they map to a letter; so 11 decimal is A, 12 decimal is B, etc. So to get 31 decimal into hex, divide by 16 -- which is the "tens place" in hex -- you get 1 with a remainder of 15 decimal, and 15 decimal is F in hex, so 31 decimal is 0x1F. Sorry about the long winded lecture. 0x19 is 25 decimal.
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"I'm a minor character in my own story", Steve Coogan as Tony Wilson in 24 Hour Party People