Ah, that makes sense. I originally read it differently.

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

Which I read as:

> Mary is 24 years old. When Mary was Ann's current age, Anne was half the age she currently is.

Which would mean Anne is 16 (because when Mary was 24-8=16, Anne's current age, then Anne was 16/2=8, half Anne's current age).

But re-reading it, then for that to be true the original would have needed to be phrased:

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