Answer:
21
Put the numbers in order from least to greatest
Cross numbers out from both sides until you get to the middle value(s)
7, 8, 15, 18, 20, 22, 25, 27, 28, 30,
8, 15, 18, 20, 22, 25, 27, 28
15, 18, 20, 22, 25, 27
18, 20, 22, 25
20, 22
When you end up with 2 numbers you add them then divide by 2
20+22=42
42÷2=21