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Ascending numbers

Source: Brazil Undergrad MO - Galois-Noether 2018 #16

November 26, 2019

Problem Statement

A positive integer of at least two digits written in the base 10 10 is called 'ascending' if the digits increase in value from left to right. For example, 123 123 is 'ascending', but 132 132 and 122 122 is not. How many 'ascending' numbers are there?
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