def calculate_average(numbers):
    total = sum(numbers)
    length = len(numbers)
    average = total / length
    return average

# مثال استفاده از تابع:
list_of_numbers = [3, 5, 7, 9, 12]
result = calculate_average(list_of_numbers)
print("میانگین اعداد لیست = ", result)