#math trivia for #April26: #117 is one of 40 day-numbers divisible by 9 this year. How many also have digits that add up to 9?
— Burt Kaliski Jr. (@modulomathy) April 26, 2012
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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