Suppose identical tags are placed on both the left ear and the right ear of a fox. The fox is then let loose for a period of time. Consider the two events and

. Let , and assume C1 and C2 are independent events. Derive an expression (involving ) for the probability that exactly one tag is lost, given that at most one is lost (“Ear Tag Loss in Red Foxes,” J. Wildlife Mgmt., 1976: 164–167). [Hint: Draw a tree diagram in which the two initial branches refer to whether the left ear tag was lost.]