Solution

ID: past-exam-of-the-mathematics-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2012/paper-2/18/d/solution

Let denote a green pear on the first draw and a red apple on the second. The two categories are disjoint, so after there are still three red apples among seventeen remaining fruits. Thus . Using from (c), conditional probability gives
Equivalently, conditioning on the second fruit being one particular category removes a red apple and leaves the other seventeen fruits symmetrically eligible for the first position.

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