How many women are there?
select count ( * ) from people where is_male = 'F'
And men?
select count ( * ) from people where is_male = 'T'
How many people are there over the age of 30?
select count ( * ) from people where age > 30
How many of them are women?
select count ( * ) from people where is_male = 'F' and age > 30