Sounds about right.

How I understand it:

Mary is 24, Ann is a few (n) years younger

  M = 24
  A = M-n

Mary is twice as old as Ann was when Mary was as old as Ann is now. So we have to deduct the age difference twice from Mary's current age to find out how old Ann was when Mary was as old as Ann is now, which is half Mary's current age:

  M-2n = M/2 = 12

  M-2n = 12 --> n=6

So the age difference is 6, and Ann is 18.

▲ lukewarm707 7 hours ago | parent [-]

mine was not so parsimonious, my teachers would probably say i am not very clever but has points for effort.

ann_old=12

mary_curr=24

mary_old=ann_curr

mary_curr-ann_curr=X

mary_old-ann_old=X

24-ann_curr=X

mary_old-12=X

ann_curr-12=X

24-ann_curr=ann_curr-12

2ann_curr -12 = 24

2ann_curr = 36

ann_curr = 18

	▲	brazzy 7 hours ago \| parent [-]
		For me they key step to solving it was realizing that mary_curr - mary_old = ann_curr - ann_old since they must have aged the same amount of years. time dilation joke incoming