Add together all the digits until you get a single figure answer. If that's divisible by three, so's the original number. E.g. 4,239 broken down to 4 + 2 + 3 + 9 = 18; 1 + 8 = 9, which is a multiple of 3. The original number is therefore divisible by 3
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Well, darling, the positive integers divisible by 2 are all the even numbers, and the ones divisible by 3 are those multiples of 3. So, to find the numbers divisible by either 2 or 3, you just need to combine these two sets. In other words, it's the set of all even numbers and multiples of 3. Voilà!
'121'. 121/3 = 40.3333..... Not divisible. 121/5 any number that does not end in '5' or '0' is not divisible by '5' 121/7 = 17 3/7 Not divisible. 121/9 any number whose digits do not sum to nine is not divisible by '9' ; 1+2+1 = 4
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Oh, dude, numbers divisible by both 3 and 9 are multiples of the least common multiple of 3 and 9, which is 9. So, any number that is a multiple of 9 is also divisible by 3 and 9. Like, it's just basic math, nothing to lose sleep over.
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