Permutations & Combinations Practice (Free)
Free permutations and combinations practice: auto-generated nPr and nCr problems with instant feedback and step-by-step solutions.
Other Topics Puzzle – Challenge 98
Other Topics Puzzle – Challenge 98 asks how many ways six people can sit in a line if two of them cannot sit together. The quick route is to count all the arrangements, then subtract the ones where the pair is adjacent — treating that pair as a single block.