Everyday Math

There are three kinds of average and they can tell very different stories. Here is when each one is honest and when it misleads.

GPA turns letter grades into a single number using grade points and credit hours. Here is the exact method, plus weighted vs unweighted.

Rounding keeps numbers readable; significant figures keep them honest. Here is how to do both without introducing errors.

Probability and odds describe the same chance two different ways. Here is how to convert between them and avoid the gambler’s fallacy.

Ratios compare quantities; proportions scale them. Here is how to use them for recipes, maps, and mixing anything correctly.

You never need an app to tip. Here are the fast mental tricks for 10%, 15%, 18%, and 20% on any bill.

Three ideas cover almost every percentage you meet in daily life. Master them and you can check a receipt or a sale price in your head.

Counting days sounds easy until months and leap years get involved. Here is the reliable method, plus the inclusive-vs-exclusive trap that catches everyone.

Conversion is just multiplying by a well-chosen form of 1. Learn that trick and a handful of anchor values and you can convert almost anything.

Working out an exact age sounds trivial until leap years and month lengths get involved. Here is the method humans use and why software does it differently.