A couple of comments:

1) You can eliminate your Select Case and just do this

average = ((a(1) + a(2) + a(3) + a(4) + a(5) + a(6) + a(7) + a(8) + a(9) + a(10) + a(11) + a(12) + a(13) + a(14) + a(15) + a(16) + a(17) + a(18) + a(19) + a(20) + a(21) + a(22) + a(23) + a(24) + a(25)) / (25 - cmbnumbers.ListIndex)

2) You can remove Call test(average) since you do it again with letter = test(average). As a matter of fact you can just do the following if you want and eliminate the second call to test as well.

lblmpaverage.Caption = average & " [" & test(average) & "]"