M4.6.3 For the adult
The OR rule and the AND rule
A coin lands heads with chance 1/2 and a dice lands on 6 with chance 1/6. What is the chance of both?
What this chapter teaches
Add the chances for "A or B" only when the events cannot both happen. Multiply the chances for "A and B" only when the events are independent. Each rule has its condition, and checking it is the point of the chapter.
Three questions to ask
There is no one right wording. Listen for the idea.
- Why can you add the chances of red and blue on a spinner, but not of "even" and "more than 3" on a dice?
- What does it mean to say two events are independent? Can you give an example?
- A coin has landed heads four times. What is the chance of heads next, and why?
After the chapter, the learner can
- Add probabilities for events that cannot both happen
- Multiply probabilities for independent events
What they copy into their notebook
A or B: add, but only if they cannot both happen. A and B: multiply, but only if they are independent. A coin has no memory.