编写一个程序，计算一个数字列表中的最大值、最小值和平均值。
数字列表：[12, 18, 6, 24, 15, 9, 21]
以下是Python编程语言的程序代码实现：

```python
numbers = [12, 18, 6, 24, 15, 9, 21]

max_number = max(numbers)
min_number = min(numbers)
average = sum(numbers) / len(numbers)

print("最大值是:", max_number)
print("最小值是:", min_number)
print("平均值是:", average)
```

程序输出：

```
最大值是: 24
最小值是: 6
平均值是: 15.0
```