Ann is 18.

I think what makes it confusing is the "variable overloading" referring to both the current ages of Mary and Ann (M & A), and their ages at some point in the past "when Mary was as old as Ann is now" (M' & A').

So, what we're given is:

M = 24

M' = A

M = 2A' => A' = M/2 = 12

Since the age gap between Mary and Ann is constant, we know:

M - A = M'- A'

So, substituting in the known values:

24 - A = A - 12

2A = 36

A = 18

We can double check the result. Since Mary (24) is 6 years older than Ann (18), then when Mary was 18 Ann would have been 12, so Mary is now twice that age as given.