※ 引述《heyhey1025 (heyhey)》之铭言： : Suppose that a person plays a game in which his score must be one of the 5 : numbers 1,2...,5 and that each of these 5numbers is equally likely to be : his score. The first time he plays the game, his score is X. He then : continues to play the game until he obtains another score Y such that : Y大於等於X. Assuming that all plays of the game are independent. Find the : probability of the event that Y = 5 : 答案是137/300 : 请帮一下 谢谢 如果一开始抽到 1 (机率1/5)，之後抽到1～5都会结束游戏，以5结束的机率是 1/5 如果一开始抽到 2 (机率1/5)，之後抽到2～5才能结束游戏，以5结束的机率是 1/4 ... 如果一开始抽到 5 (机率1/5)，只有之後抽到5才能结束游戏，以5结束的机率是 1/1 1 1 1 1 1 1 137 - * [- + - + - + - + - ] = --- 5 5 4 3 2 1 300 -- --

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