Standard patterns
A standard method from Permutations and combinations: the steps.
Steps
| Situation | Method |
|---|---|
| A and B must be together | glue them into one block; arrange the blocks; × 2! for the order inside |
| A and B must not be together | total − together |
| at least one … | total − none |
| particular person must be included | put them in; choose the rest from the others |
| first and last positions fixed | fill those first, then the middle |
For the whole topic, read Permutations and combinations: key points, common mistakes and how the DSE tests it.