Question 257629
http://mathforum.org/library/drmath/view/57277.html
from that website 
"Now, if Pat is 24, then Chris was 12 (half Pat's current age) at some 
point in the past. That point was when Pat was as old as Chris is now.  

Restating it, some number of years ago - let's call it x - Chris was 
12 and Pat was Chris's current age."
24-x=12+x
12=2x
6=x
24-6=18 Chris's age
12+6=18