What Back-of-the-Napkin Calculation Should Everyone learn?
What are some of the most powerful and useful but easy calculations should more people know how to use?
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.
If something has a 1/n chance of happening and I try it n times, there's only a 63% chance of it happening.
(Approaches 63% as n gets larger)
15% of 33 is the same as 33% of 15. Sometimes you get an easier calculation by switching the numbers this way.
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.
1 year ~= PI 10^7 seconds
Earth's orbital diameter ~= 1000 light-seconds
The PIs cancel!
So Earth's orbital velocity ~= 0.0001c ~= 30 km/s
The Fibonacci sequence for approximately converting kilometres to miles.
X% of Y is Y% of X
Ex:
7% of 50?
50% of 7 = 3.5
7% of 50 = 3.5
Percentage change:
((New / Old) - 1) * 100