#math trivia #117 solution

Any number whose digits add up to a multiple of 9 is divisible by 9, and vice versa.

Most of the day-numbers this year that are divisible by 9 have day-numbers that add up exactly to 9.  The ones that don’t are:  99, 189, 198, 279, 288, and 297.  Thus, 34 of the 40 day-numbers have digit-sums of 9.

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