We need to know how many students wrote more than 6 and fewer than 20 letters. Then, we count the number of x from 7 to 19:
7: 3
8: 2
9: 4
10: 0
11: 1
12: 0
13: 3
14: 2
15: 4
16: 0
17: 1
18: 2
19: 4
Then, adding these values:

Since each x represents two students, the corresponding number of students is:
