Average for range of values (VLOOKUP?)

Hi - I'm trying to get the average for a set of values distingushed from the main group by a value in a corresponding column.
Presumably I need a lookup function but can't figure out quite how to do it.
e.g.
#1 #2
a 12
a 10
a 9
b 24
b 26
c 17
c 14
a 16
Q. How do i get the average for all of the values correpsonding to a's b's and c's?

Cheers
Poolio