Author note — to cover

Add probabilities of mutually exclusive events for either-or questions, two or three alternatives, fraction and decimal answers, always checking exclusivity before adding. Both tiers. Exclude non-exclusive events (overlapping-events page) and the general addition law quoted with union and intersection notation, which is A level. (P4)

• Fluency — direct questions on the or rule for exclusive events, limited to this lesson's stated scope (4 questions).
• Application — use the same skill in unfamiliar or contextual questions without introducing a later method (4 questions).
• Check — questions built around the exclusions and likely misconceptions named in the lesson brief (4 questions).