In January 2008, I offered several links to mental math tricks. I've gathered a few more over the past months, and I thought it was time to share them with you.

I'll start with an original mental math feat I've never seen anywhere before. You have someone name a repeating decimal, for example, 0.5757575757, and you can instantly name a fraction, like 19/33 for our example, which will give that repeating decimal. Blake O'Hare's Nerd Paradise teaches this feat in his Math Funbox: Decimal to Fraction post. The rest of his posts should interest most regular Grey Matters readers, as well.

I've mentioned Wild About Math! before, and the site is still worth checking out. They offer 4 great mental math feats, each taught in a “mathcast”, which are videos done in a sort of whiteboard style. Most of them involve the quick multiplication of 2- and 3-digit numbers, yet are suprisingly easy.

While I'm thinking about Wild About Math!, they also have one post that contains 9 different mental math tricks which should be in any math geek's arsenal. This post, for obvious reasons, has been copied and posted on several different blogs, but I think the originator should be given the proper due.

I'll round out this post with gummy stuff's Vedic Math page. There are plenty of links containing the 16 principles of “Vedic Math”, but these have the clearest examples and explanations I've seen for how and why these principles work. Only 10 of the 16 tutorials on this page were completed, however, so the rest you'll have to find out about for yourself.

Does anyone else besides me think it's curious that all of these pages use lists of lengths that are square numbers?