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Mobile Numbers: Multiples of nine

In this series of posts, Katie investigates simple mathematical concepts using the Google Sheets spreadsheet app on her phone. If you have a simple maths trick, pattern or concept you’d like to see illustrated in this series, please get in touch.

We’re all (hopefully) aware that a pleasing property of numbers that are divisible by nine is that the sum of their digits is also divisible by nine.

It’s actually more well known that this works with multiples of three, and an even more pleasing fact is that the reason three and nine work is because nine is one less than the number base (10), and anything that’s a factor of this will also work – so, in base 13, this should work for multiples of 12, 6, 4, 3 and 2. Proving this is a bit of fun.

Once when I was thinking about this fact, an interesting secondary question occurred.

Follow Friday, 21/03/14

It’s been a while since we’ve done one of these, but here’s a selection of Twitter accounts you may wish to follow. This week, the theme is numbers!

1. Prime Numbers

While I usually try to pull out an interesting tweet to showcase the brilliance of the accounts I recommend, in this case the account is tweeting every prime number. It’s run by an automated script, which you can see the code for, and according to its bio, aims to tweet “Every prime number, eventually”. Ambitious.