This sounds absurd but it's true:

To tell if a number is divisible by 11, number the digits starting from the leftmost, and then add up (separately) the odd-numbered and even-numbered digits and subtract the smaller sum from the larger. If that difference is divisible by 11, the original number is.

Take 27263877454 as an example.

a = 2+2+3+7+4+4 = 22
b = 7+6+8+7+5 = 33

b - a = 33 - 22 = 11

Since 11 is, well, divisible by 11, 27263877454 is as well. (It's 11 * 2478534314.)

Follow

@noelle this is the kind of stuff we had to prove in intro to higher math

Sign in to participate in the conversation
Awoo Space

Awoo.space is a Mastodon instance where members can rely on a team of moderators to help resolve conflict, and limits federation with other instances using a specific access list to minimize abuse.

While mature content is allowed here, we strongly believe in being able to choose to engage with content on your own terms, so please make sure to put mature and potentially sensitive content behind the CW feature with enough description that people know what it's about.

Before signing up, please read our community guidelines. While it's a very broad swath of topics it covers, please do your best! We believe that as long as you're putting forth genuine effort to limit harm you might cause – even if you haven't read the document – you'll be okay!