Name the two first-branch events and their conditionals, and the whole tree draws itself: every path probability, the marginal for each second-stage outcome, and the famous flip — P(A | B) computed from the same picture, in reversed order. Frequency view (“out of 1,000”) included, because it turns exam-trick conditional questions into simple counting.
Right-hand second-stage branches are automatic complements (1 − q).
Keep probabilities between 0 and 1 — the fields clamp and tell you when they don't.
| path | product | probability | per 1,000 |
|---|