There are 18 people on the football team.
30 - 10 = 20 + 1 = 21 jerseys available.
21 - 3 jerseys extra = 18 people wearing jerseys.
When we deduct 10 from 30, we need to add back 1 because jersey number 10 was not included when we got the difference.
10, 11, 12, 13, 14, 15, 16
17, 18, 19, 20, 21, 22, 23
24, 25, 26, 27, 28, 29, 30