I Can Never Remember ''divide By Five'' But I DO Have The Speed Of Sound Memorized - Tumblr Posts

Another fun math tip: the speed of sound in air in ambient conditions is almost exactly a third of a kilometer (343 m) per second. The old trick of counting seconds between the flash of lightning and the associated thunder and dividing by five to get the distance in miles is handy but imprecise, and it's hard to divide numbers by five offhand if they're not directly divisible. I like to use thirds-of-a-kilometer instead.

Fun little math trick I find really helpful: the ratio of a mile to a kilometer is within 1% of the Golden Ratio. That means that if you have a good memory for Fibonacci numbers (1 2 3 5 8 13 21 34 55 89) you can convert pretty accurately by taking consecutive Fibonacci numbers.

For example, 89 kilometers is really close to 55 miles (55.3). Or, say you need to convert 26 miles to kilometers: 26 can be written as 21 plus 5, so taking the next Fibonacci number up gives 34 and 8, meaning it should be around 42 kilometers. Sure enough, it's 41.8 km!