IXL - Basic Operations Practice
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Skill-by-skill practice site from IXL Learning covering third-grade math, including multiplication and division facts, multi-digit addition and subtraction, and word problems. Students get instant feedback and explanations; free use is capped at a few questions daily without a subscription.
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Textbook from the American Mathematical Society for prospective elementary teachers, rebuilding whole numbers, integers, and decimals from their foundations using the number line. Readers learn to justify standard algorithms and explain why arithmetic works, with applications and geometry topics.
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