Solution

ID: past-exam-of-the-year-1-of-the-computer-science-course-of-the-university-of-oxford/2016/discrete-mathematics-continuous-mathematics-and-probability-resit/7/a/ii/solution

Because the partition the sample space, the events are disjoint and their union is . Finite additivity and the definition of conditional probability give the law of total probability
The assumption ensures that each conditional probability is defined.

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