mine was not so parsimonious, my teachers would probably say i am not very clever but has points for effort.
replyann_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
For me they key step to solving it was realizing that
replymary_curr - mary_old = ann_curr - ann_old
since they must have aged the same amount of years.
time dilation joke incoming