As far as I can see the function works correct.

Values in A1 to A7:
1

3
5

6
7
=average(A:A) gives 4.4 which seems correctly.

For the second problem if you want results only for numeric data in A1
and B1 use this

=IF(AND(ISNUMBER(A1),ISNUMBER(B1)),B1/A1,"")

Hans