Numbers with Ascending Digits

Miri writes down all the numbers in whose decimal representation only the digits `1, 2, 3, 4, 5` appear.
(Not all digits have to appear) and all the digits that appear are written in ascending order (e.g., 135 or 1234 or 5 but not 153 and not 1223).
How many numbers will Miri write down?

Difficulty level (1 very easy - 10 very hard): 1

Topics:
Combinatorics
Sources:
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