Problem:
A number is upwards if its digits in base 10 are nondecreasing when read from left to right. Compute the number of positive integers less than that are both upwards and multiples of 11.
Solution
Solution:
For a number (allowing leading 0s) to be upwards and a multiple of 11, we must have
Note that , , and are all nonnegative. Thus,
Therefore,
which can only occur when , , and , i.e. the number is of the form . We can easily verify that all numbers of the form for digits satisfy our conditions, so we simply have to count them.
There are such triples of digits . However, one of these triples is , which corresponds to the number 0. Thus our answer is .
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