Puzzleup 2013 (5) Pool Of Problems - 拼圖

題目網址: http://www.puzzleup.com/2013/
http://www.puzzleup.com/2013/puzzle/?246
答題時限: ８月２２日７ＰＭ－比賽結束（約12月11日）
加分時限: ８月２２日７ＰＭ－８月２６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）　　

◆Pool Of Problems

Using a pool of problems, 20 tests will be formed.
-Every test should have the same number of problems.
-Any problem should be included in at most 10 tests.
-For every 5 tests, there should be at least 2 problems common to all of them.
What can be the minimum number of problems in this pool?

你必須從題庫中出題設計二十份考題。

- 每份考題都要有一樣多的題目
- 每道題目最多在十份考題中出現
- 任意五個考題都至少要有兩題一樣的題目

請問滿足這個條件的題庫至少要有幾題?

--