SQl -查找每个人的平均数量。

I have two tables

我有两个表

class
| id      | area       | students  |
|       1 |      area1 |     2     |
|       2 |      area1 |     28    |
|       3 |      area1 |     22    |
|       4 |      area2 |     4     |

deliveries
| id      | kg         | classid   |
|       1 |      120   |     1     |
|       2 |      80    |     1     |
|       3 |      20    |     1     |
|       4 |      200   |     2     |
|       5 |      150   |     3     |
|       6 |      14    |     2     |
class
| id      | a