Average questions

IMO C

ok, the purpose of my question is to get a clear demonstration not only the answer.

Xbond:
here is my explanation with examples
Case 1:insufficient
consider these set of numbers:

28,29,31,32
28,29,30,33

you see you can't tell how many are greater than 30
Case 2:

9,10,50,51
9,10,1,100
you still can't say

Combine both
9,10, the third number lets say is 30. Now to have an average of 30 you must have the 4th number 61 which violates the condition in 1. so the third number has to be at least 31

gotcha!!:))
its C. there are two numbers greater than 30

many thks