Let x = Jane's age now.
Jane was 12 when Jessica was Jane's present age, x.
Let y = the number of years ago it's been since Jane was 12
and Jessica was x. 
Jessica's age y years ago = Jane's age now.
                   24 - y = x
   Jane's age y years ago = 12
                    x - y = 12
So we have this system:
Can you solve that system?  If not post again asking how,
x=18, y=6
So Jane is 18 years old now.  
Checking:  y=6 years ago, Jane was 12, and Jessica was 18. Today
Jessica is twice 12, Jane's age then.
Edwin