There are 100 questions in a certain exam, each question is worth one point, and no points will be deducted for mistakes. Students A, B, and C get 90 points, 70 points, and 50 points respectively. The question that all three of them can solve is called "easy ", only one person can solve the problem is called "more difficult", and no one can solve the problem is called "extremely difficult", and the "more difficult" is three times the "extremely difficult", and it is known that classmate C can do More than 80% of the questions are "easy questions", but not all are "easy questions". May I ask: How many "special difficult questions" are there?
某次考试共有100道题，每题一分，做错不扣分，甲、乙、丙三位同学分别得90分、70分、50分，其中3个人都做出来的题叫作"容易题"，只有1个人做出来的题目叫作"较难题"，没人做出来的题目叫作"特难题"，且"较难题"是"特难题"的3倍，又已知丙同学做出的题中超过80%的是"容易题"，但又不全是"容易题"，请问："特难题"共有多少道？
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