select avg(N.number) as Average
from (
    select t.team_name, count(tt.team_id) as 'Number'
    from teams as t
    left join (
        select t.team_id, t.TEAM_NAME
        from teams as t
        join employees as e on e.TEAM_ID= t.TEAM_ID
        where e.gender = 'F'
    ) as tt on t.TEAM_ID = tt.team_id
    group by t.team_name
) as N