let say present age of Chris is x and it is given Pat's age is 24
Also say t yrs ago , Pat was x yrs old i.e. 24-t=x i.e. t=24-x
Hence Pat was x yrs old ,(24-x) yrs ago.At that time he was twice as old as Chris i.e. Chris must have been x/2 yrs old , (24-x)yrs ago i.e.
x - (24-x) = x/2
Solve for x, x=16 i.e. Present age of Chris is 16