从一组随机数字中找到累积平均值

Question

1. 编写一个Python程序来模拟给定课程中N名学生的期末考试成绩。用户将提供N的值。仅打印成绩的分布，有多少学生获得A，B，C，D和F。

2. 修改＃1中的程序，以对N个学生运行该程序M次。 M次后，为每个字母等级打印累计平均值。用户将提供M的值。

作为班级实验任务的一部分，我最终承担了以下责任：

#2 NEED Cumulative Average(#1 modified to run M number of times)
import random
N = int(input("Please enter the number of students:"))
M = int(input("How many times would you like to run this?"))
tries = 0
countF = 0
countD = 0
countC = 0
countB = 0
countA = 0
while tries != M:
  tries = tries + 1
  for i in range (N):
    score = random.randint(30,100)
    if score <= 50:
      grade = "F"
      countF = countF + 1
    elif score <=69:
      grade = "D"
      countD = countD + 1
    elif score <= 79:
      grade = "C"
      countC = countC + 1
    elif score <= 89:
      grade = "B"
      countB = countB + 1
    else:
      grade = "A"
      countA = countA + 1
print(countF, countD, countC, countB, countA)

虽然此程序在＃1上可以正常工作，但我很难确定＃2的累积平均值的编码。诚然，我的数学缺乏，所以我可能看不到简单的答案。任何帮助表示赞赏。 谢谢。