Core idea
nPr counts ordered selections; nCr counts unordered selections. nCr=nPr/r!. Define whether order matters before choosing.
Worked check
From 5 books choose and arrange 2: 5P2=20; choose 2 without order: 5C2=10.
Practice
- Show each step and state any domain or assumption.
- Check the result by substitution, estimation or a second method.