What you'll learn

32 lessons in Arithmetic

Place valueAddition & subtractionMultiplicationDivisionOrder of operations (PEMDAS)Fractions: meaning & equivalenceAdding & subtracting fractionsMultiplying & dividing fractionsDecimalsPercentsNegative numbersEstimation & mental mathDivisibility rulesGCD, LCM & the Euclidean algorithmModular arithmetic & casting out ninesExponents, roots & scientific notationRatios, proportions & unit ratesBinary & other number basesRepeating decimals ↔ fractionsMixed repeating decimalsFast mental multiplicationTriangular numbers & Gauss summationPrime factorization & the fundamental theoremFraction–decimal–percent conversionsSimple interestCompound interestUnit conversion & dimensional analysisRounding & significant figuresFermi estimationEstimating square roots by handPercent applicationsRate & unit-rate word problems

How Erudia teaches

Built to be understood — and remembered.

Every idea is taught with motivation and a worked example before the drills, and an FSRS spaced-repetition engine schedules each review for the day its model predicts you would forget it. A short placement check finds what you already know, so you start Arithmetic exactly where it's useful.

Start Arithmetic →