没问题，我来一步步帮你解决这两个问题：

要实现总数统计，核心思路是初始化一个计数器变量，然后遍历目标数据集，用if语句判断元素是否符合你要统计的条件，每满足一次就把计数器加1。

举个简单的Python示例，比如统计列表中大于10的数字数量：

numbers = [5, 12, 8, 15, 3, 20]
count = 0

for num in numbers:
    # 用if判断是否符合统计条件
    if num > 10:
        count += 1

print(f"大于10的数字总数是: {count}")

这里count就是我们的计数器，每次if条件成立（数字>10），就给count加1，最后输出总数。

假设我们有两个列表，分别存储两门课程的学生名单，比如cs_class（计算机科学课）和math_class（数学课）。我们可以用嵌套循环遍历两个列表，判断同一个学生是否同时出现在两个列表里，同时记录符合条件的学生并统计总数。

代码示例如下：

# 假设的两门课程学生列表
cs_class = ["Audrey", "Ben", "Julia", "Paul", "Sue", "Mark", "Lily"]
math_class = ["Audrey", "Ben", "Julia", "Paul", "Sue", "Mark", "Tom"]

common_students_count = 0

# 嵌套循环：遍历计算机课的每个学生
for cs_student in cs_class:
    # 内层循环遍历数学课的每个学生
    for math_student in math_class:
        # 判断是否是同一个学生（同时选修两门课）
        if cs_student == math_student:
            print(f"Student: {cs_student} is enrolled in both classes")
            common_students_count += 1
            # 找到匹配后跳出内层循环，避免重复统计
            break

print(f"{common_students_count} students are enrolled in both Computer Science and Math")

运行这段代码后，输出就会和你给出的示例一致：

Student: Audrey is enrolled in both classes
Student: Ben is enrolled in both classes
Student: Julia is enrolled in both classes
Student: Paul is enrolled in both classes
Student: Sue is enrolled in both classes
Student: Mark is enrolled in both classes
6 students are enrolled in both Computer Science and Math

补充一句：实际开发中用集合的交集操作会更高效，但既然要求用嵌套循环，上面的代码完全符合需求。

内容的提问来源于stack exchange，提问作者Sammy