10. Arrange 21 students from class 3 (1) with student numbers from 1 to 21 in a circle. Record the difference between the student numbers of all adjacent pairs by subtracting the smaller number from the larger one. What is the maximum possible sum of these 21 recorded differences? $\qquad$ -
```
            [Analysis] The problem requires the sum of "differences" to be maximized. To achieve this, the "difference" must be large, so two adjacent numbers cannot be together. Large numbers and small numbers should alternate;
            21, 1, 20, 2, 19, 3, 18, 4, 17, 5, 16, 6, 15, 7, 14, 8, 13, 9, 12, 10, 11
            Since this forms a circle, we also need to add the difference between 21 and 11, which is 10.
            The sum of the arithmetic sequence 1+2+3+...+20:
= (1+20) \times 20 \div 2
= 210
        210 + 10 = 220
```