Average based on certain data

Hello, I need your help.

I need to make an average based on certain data.

I have a table with two columns. The first column (A) contains repeating numbers.

in the second column (B), numbers are assigned based on the first column.
for example:

A - B
1 - 58
2 - 67
3 - 80
1 - 45
1 - 78

If I choose number one, it must make an average from numbers 58, 45, 78...

If I choose number 2, it must make average from number assigned to number 2 etc...

Thank you!