4 ms·
Ask HN: 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?
- throwaway888abc 6y agohttps://www.amazon.com/Back-Napkin-Expanded-Problems-Pictures/dp/1591842697 https://www.amazon.com/Back-Napkin-Expanded-Problems-Picture...
- helph67 6y agoThe Pareto Principle, simple but important to know! https://en.wikipedia.org/wiki/Pareto_principle https://en.wikipedia.org/wiki/Pareto_principle
- Jtsummers 6y agoDoubling 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.
- giantg2 6y agoI wish I could upvote this more than once.
- schwartzworld 6y agoIt 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.
- musicale 6y agoSo the Indiana legislator who wanted a simpler value for pi was on to something...
- schwartzworld 6y agoI 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.
- yen223 6y agoQuick 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.
- deleted 6y ago[deleted]
- Imanari 6y ago15% of 33 is the same as 33% of 15. Sometimes you get an easier calculation by switching the numbers this way.
- giantg2 6y agoDoubling 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.
- cutthegrass2 6y agoI can't explain how incredibly satisfying it was to fully grasp KE=1/2mv2. Twice as much initial velocity very clearly has 4x the area under the curve on a speed/time graph.
- ryanchants 6y agoIf 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)
- xpe 6y agosee also: https://math.stackexchange.com/questions/1799767/63-chance-of-event-happening-over-repeated-attempts/1799784 https://math.stackexchange.com/questions/1799767/63-chance-o...
- HAL9OOO 6y agoX% of Y is Y% of X Ex: 7% of 50? 50% of 7 = 3.5 7% of 50 = 3.5
- mikhailfranco 6y ago1 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
- theriddlr 6y agoThe Fibonacci sequence for approximately converting kilometres to miles.
- catacombs 6y agoPercentage change: ((New / Old) - 1) * 100