Puzzleup 2011 (04) Code Length - 拼圖

題目網址: www.puzzleup.com/2011/puzzle/?222
加分時限: 8/18 7PM - 8/23 7PM
答對可得基本分１００分。答案可上傳５次，每改１次答案從基本分扣２０分。　
比賽期間內可隨時上傳答案，加分時限內答對第ｎ天加（６－ｎ）分　 　　 　　
另依題目的難易有額外加分（如有８０％的人這題答錯，答對者加８０分）　　

◆Digital Watch

Each of the 26 letters of the English Alphabet are written on a card.　 　
All 26 cards are put in a bag. Cards are drawed from the bag randomly
and put together to form codes. Each code must contain three vowels. 　
When third vowel is drawn, we get a legitimate code and stop drawing cards.

The opearation is repeated for a long time. What is the expected mean
codelength?(The letters A, E, I, O and U are considered as vowels for
this question)

Note: If needed, use dot as decimal separator, and decimals with up to
five places. For example: 12.34567
　
英文２６個字母各寫在２６張卡片上放入袋中，從袋中隨機抽取卡片成為密碼。
每個密碼須有３個母音，抽到第３個母音表此碼已合理，停止抽卡。　　

這樣操作了很長一段時間，問密碼均長的期望值為何？（母音為 A, E, I, O, U。）

注意: 如需要可用小數作答，最高可答至小數點後５位。例：12.34567

－－
驚覺今年programup都開始一個月了，版上都沒消息
是太簡單了所以大家火花掉嗎@ _@a
不過滿適合文組的復習一下概念
至少我算得挺開心的 哈



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沒事多喝水
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