Here's the simplest form I could distill it to. The other comments I find quite verbose or confusing, no offense intended.

    x: Ann's current age
    y: The age difference between Ann and Mary

    24 = x + y      (ie. their current ages)
    x = (24/2) + y  (ie. their age in the past)

    Solve for y in one equation and plug it into the other. You'll get x = 18.

That's not how I read it. I read it as:

  Now : Mary is 24 = 2y,  Ann is x
  Past: Mary is x,        Ann is 12 = y