Solution

ID: past-exam-of-the-mathematics-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2025/paper-2/12/d/solution

Let and denote the first and second labels. Sampling with replacement restores the bag before the second draw, so conditioning on only specifies the threshold. For , there are qualifying green balls and qualifying red balls. For , every green ball and the first red balls qualify. Hence the requested conditional probability is
The formulas agree at and give one at . Labels outside the possible first-draw range do not define a conditioning event of positive probability.

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