This is a trick that everyone should learn in the 3rd grade. If not, then at least by the 9th grade.

If you’re in the dark on this one, no big deal. Now’s the time to learn. If you’re a parent – this trick will win you points like crazy. Especially if you get to it before the teacher does.

So here’s how it works. For both 3 and 9, start by adding up the digits in the number. If the result is divisible by 3, then so’s the original number. If the result’s divisible also by 9, then… you guessed it… so’s the original number.

If your result is way to big to tell, do it again. Check out the videos for both:

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