Doubling rate based on percentage. Roughly: 72 / percentage = years to double. An investment returning 1%/year will double after 72, 6% will double after 12. And for debts, illustrates for people who aren't good with money why that 20% (or worse) CC rate is so bad.
Doubling your speed quadruples your stopping distance for most cars. This is useful for all drivers to understand from a conceptual level, even if they aren't crunching the numbers.
There is a similar one useful for RF - doubling your distance quadruples the signal loss. Probably only useful for hams or people setting up wireless networks. But I think it can be good to understand this so that you can reduce your RF exposure for health precautions.
Quick way to get the median of some value - take 5 random samples. There will be a 92.5% chance the median is between the largest and the smallest value.
It blew my mind when somebody pointed out to me that pic is close enough to 3 that you can just consider it to be 3. You can figure out the area and circumference of any circle in your head for the most part.
I mean, I'd still want engineers using the real value in important situations, but that margin of error is acceptable for any circle I need to measure.
