Classes A, B, and C have 30, 40, and 20 students respectively. The average score of the three classes in the math test is 92. It is known that the average score of Class A is higher than the total average score of Class B and Class C. 9 points, the average score of class A is 8 points higher than the average score of class B, what is the average score of class C?
甲，乙，丙三个班分别有30人，40人，20人，三个班参加数学考试的平均分为92分，已知甲班的平均分比乙班和丙班的总平均分高9分，甲班的平均分比乙班的平均分高8分，求丙的平均分是多少？
<result>87</result>