翻牌：先集齐水土且避开火
Four cards are shuffled
题目详情
Four cards are shuffled and placed face down in front of you. Their faces (hidden) display the four elements: water, earth, wind, and fire. You are to turn the cards over one at a time until you either win or lose. You win if you turn over water and earth. You lose if you turn over fire. What is the probability that you win?
解析
风牌不影响胜负，可忽略。等价于只看三张牌 的相对顺序。
你赢当且仅当“火”在这三张中排最后（因为在翻到火之前必须先翻到水和土）。
三张的相对顺序等可能，共 6 种，其中火最后的有 2 种（WT F 或 TW F），概率为