Guides
The reasoning behind the numbers — clear, worked explanations.
From finding the percentage of a number to percentage increase and decrease — the four questions, formulas and worked examples.
What body mass index measures, the WHO categories, the healthy weight range, and what BMI does and doesn't tell you.
The amortization (annuity) formula behind every fixed monthly payment, with a fully worked $10,000 example.
Adding a month to January 31st doesn't give you "February 31st" — here's the order calculations actually happen in, and why it matters.
Percent change picks one value as the reference and is direction-sensitive. Percent difference treats both values equally — and gives the same answer no matter which order you enter them.
C(n, r) counts selections where order doesn't matter — like lottery numbers. The moment order matters, you need permutations instead, and the count changes dramatically.
Counting down to a deadline, trip or event means finding the day-level difference between today and a target date — direction and whether to include today both matter.
The Devine formula behind most "ideal weight" calculators comes from 1970s medication-dosing practice, not a public health guideline — here's what it actually measures.
Slope, the line equation, the angle of incline and the distance between two points all come from the same two coordinate pairs — here's how each is derived.
All four circle measurements can be derived from just one of them — here's how, with a worked example going the "reverse" direction from area back to radius.
Subtracting two clock times seems trivial until the end time is earlier than the start — like an overnight shift crossing midnight.
The two formulas differ by just one number in the denominator (n vs. n−1) — but picking the wrong one systematically biases your result.
A cone is always exactly one-third the volume of a cylinder with the same radius and height — and a sphere with matching dimensions sits exactly between the two.