Arithmetic

Learn Arithmetic

Where it all starts: place value, the four operations, fractions, decimals, and percents. Everything later is built on top of this.

Start Arithmetic free → Browse all subjects

Free to start · adaptive placement finds your level · reviews timed so it stays learned.

What you'll learn

27 lessons in Arithmetic

Place valueAddition & subtractionMultiplicationDivisionOrder of operations (PEMDAS)Fractions: meaning & equivalenceAdding, subtracting, multiplying, dividing fractionsDecimalsPercentsNegative numbersEstimation & mental mathGCD, LCM & the Euclidean algorithmExponents, roots & scientific notationRatios, proportions & unit ratesBinary & other number basesRepeating decimals ↔ fractionsModular arithmetic & casting out ninesFast mental multiplicationTriangular numbers & Gauss summationPrime factorization & the fundamental theoremDivisibility rulesFraction–decimal–percent conversionsSimple & compound interestUnit conversion & dimensional analysisRounding & significant figuresFermi estimationEstimating square roots by hand
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 moment just before you'd forget it. A short placement check finds what you already know, so you start Arithmetic exactly where it's useful.

Start Arithmetic →