One of four different prizes was randomly put into each box of a cereal. If a family decided to buy this cereal until it obtained at least one of each of the four different prizes, what is the expected number of boxes of cereal that must be purchased?
몇번사는지도 안나와있고 그냥 1/4 로 확률이 동등하게 상이 나눠져있는건데 적어도 하나씩얻을때까지 구매할 개수의 대한 기댓값?

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답은 25/3임