Presh Talwalkar's Mind Your Decisions blog, has many interesting posts, some of which I've covered before.

I was thrilled recently when he began to make a series of videos about various mental math techniques!

It seemed to start about 2 months ago, when he posted the following video about squaring numbers ending in 5:



At the same time, he posted a companion video explaining why this trick worked:



Apparently, they've proved popular, as he's released a small series of them so far! Just like the above pair, each lesson provides a video teaching the method and an explanation behind the method. This is a good approach, as some may just want to impress their friends and family with a technique, without worrying about why it works, can just learn and go, while those who are more curious can take the extra step in learning about it.

Among the more unusual methods taught is the following method for squaring 34, 334, 3334, etc. and 67, 667, 6667 (explanation video):



A more standard technique is taught in this video about multiplying any 2 numbers between 11 and 19 (explanation video):



Presh Talwalkar has also made several short mental math videos that each teach simple techniques and includes simple explanations along the way. The first of these was a video on the rule of 72, a way of calculating how long it will take an investment to double:



Another one was this handy lesson in calculating percentages in your head:



My favorite of this group, however, is the following video on converting decimal numbers to binary in your head, as I've managed to impress even well-posted computer programmers with this feat:



If you enjoyed these videos, take a look around the rest of the Mind Your Decisions blog. Presh posts math puzzles every Monday, game theory posts every Tuesday, and other posts as thing grab his interest.