The average severity is calculated by taking the total sum of all assets (hosts and web applications) severity scores (CVSS score) and then divide this sum with the number of assets concerned.
There are 3 assets with the following CVSS score:
- Asset 1: 7.0
- Asset 2: 4,1
- Asset 3: 9,0
This gives a total score of 20,1. The average severity is 20,1/3 = 6,7.