This may help people:

> Mary is 24 years old. She is twice as old as Ann was (12) when Mary was as old (18) as Ann is now (18). How old is Ann?

So 6 years ago, Mary was 18, and Ann was 12.

Today, Mary is twice the age that Ann was (12 * 2) at the time that she (Mary) was 18, making her 24.

What makes it confusing is that the sentence is comparing Mary’s present age with Ann’s past age (which I did not catch until mulling it over a bit).