Mind Performance Hacks

Hack #36. Make Friends with Numbers

With a little experience, you can learn to recognize many individual numbers by their special properties. Some of these properties can help you with mental arithmetic and memory.

Just like a face in a crowd, a number such as 1,729 probably doesn't mean much to you. But 1,000,000 looks like a friendly face. With some effort, more numbers can look like friends. Here's how to get started making their acquaintance.

In Action

Let's start with some numbers that you're probably already friendly with—10 and its powers: 100, 1,000, and so forth. Because we use a decimal number system [Hack #40], powers of 10 end in zeros. Companies are aware of that and try to get phone numbers that are multiples of a power of 10, because those numbers are easy to remember. For instance, the publisher of this book, O'Reilly Media, has a local phone number of [phone], which is a multiple of 1,000. Multiplying by a power of 10 is easy; you just have to append zeros:

314 × 1000 = 314000

Similarly, dividing by a power of 10 just removes zeros (or moves the decimal point to the left if there aren't enough zeros):

2030 / 100 = 20.3

If we look at the factors of 10, which are 2 and 5, we can come up with other useful rules. (Factors of 10 are also called aliquot parts.) For instance, to multiply a number by 5, first multiply by 10 and then take half the result. This is based on the notion that, for multiplication and division, 2 is a friendlier number than 5. For instance, 386 ...