A certain number of salesmen were appointed in different territories and the following
data were compiled from their sales reports.

Sales :
4-8
8-12
12-16
16-20
20-24
24-28
28-32
32-36
36-40

No. of salesmen :
11
13
16
14
?
9
17
6

If the average sale is 19920, find the missing value.​