khanacademy.org
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Khan Academy is a free online education platform offering extensive math courses, including combinatorics and related topics. It provides video lessons, step-by-step problem walkthroughs, and interactive practice exercises with progress tracking.
More resources on Combinations
Brilliant.org Combinatorics
Interactive lessons on combinations (free tier available)
Art of Problem Solving - Combinatorics
Free wiki with problems and solutions on combinations
Mathematics for Computer Science (includes Combinatorics)
Learn discrete mathematics and combinatorics for computer science in this comprehensive course from MIT professors Leighton & van Dijk.
Combinatorics
Master combinations with Trefor Bazett's combinatorics course! Learn essential counting techniques and problem-solving skills.
mathworld.wolfram.com
Wolfram MathWorld is a comprehensive online mathematics encyclopedia that offers clear definitions, explanations, theorems, and worked examples across all math topics. It includes in-depth articles on combinatorics, including combinations, binomial coefficients, counting principles, and related formulas and proofs.
ocw.mit.edu
MIT OpenCourseWare (OCW) provides free, openly available MIT course materials—lectures, notes, assignments, exams, and videos—across numerous subjects. You can search for math courses (including combinatorics and discrete math) and download resources to study topics like combinations.