Puzzleup 2013 (1) Eight Balls - 拼圖

一年一度的 Puzzleup 又登場了～

題目網址: http://www.puzzleup.com/2013/
http://www.puzzleup.com/2013/puzzle/?240
答題時限: ７月25日７ＰＭ－比賽結束（約12月11日）
加分時限: ７月25日７ＰＭ－７月30日６：５９ＰＭ

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）　　

◆Eight Balls

There are 8 balls. Four of them weigh X grams each, and the other four weigh
Y grams each. Your task is to find two balls having different weights. You
have a balance scale with two pans on which you can compare the weight of any
number of balls. What is the minimum number of weighings necessary to
guarantee to accomplish this task?

有八個球，其中四個各重Ｘ克，另外四個各重Ｙ克。

你的任務是找出兩個不同重量的球。

你手上有一個天平，天平的秤盤上可以比較任意數量的球的重量。

試問至少需要使用幾次天平才能達成這個任務？

--
'You've sort of made up for it tonight,' said Harry. 'Getting the
sword. Finishing the Horcrux. Saving my life.'
'That makes me sound a lot cooler then I was,' Ron mumbled.
'Stuff like that always sounds cooler then it really was,' said
Harry. 'I've been trying to tell you that for years.'
-- Harry Potter and the Deathly Hollows, P.308

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