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Author Josh Rappaport had written a blog post on the divisibility by 3 trick. If you’re not familiar with this trick, it states that by finding if the sum of the digits in a number is divisible by 3 then the number itself is divisible by 3. For instance, the number 12,345 is divisible by 3 because 1+2+3+4+5 = 15, which is divisible by 3. Taking the trick even further, the digits in 15 – 1+5 – add to 6, which is also divisible by 3! Neat stuff!
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But why does this work? I guess if I had to, and I had a bunch of time!, I may have been able to figure this out (I’d disappoint my professors if I couldn’t), but I thought I’d ask Josh for the cheat. He wrote the best blog post I’ve read in a while on WHY this trick works… How to See Why the Divisibility Trick for 3 Works. Check it out!
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Thanks, Shana! I love that “3″ also. Very stylish!
— Josh
Well, I sort of stole that 3 from the interweb. Shhhh…!