This seems difficult at first, but we need to find a system of equations that we can solve for the unknowns. What we don't know is Ellen's age now and the difference in the girls' ages.
Let x= Ellen's present age
y= difference in their ages
We know that Terry is 24, so that means that Ellen's age, x, plus the difference in their ages, y, must equal 24:
Eq. I: x+y=24
We also know that when Terry was Ellen's age now, 24-y, she was twice Ellen's age then, 2(x-y):
Eq. II: 24-y=2x-2y
Substituting the value for y from Eq. II into Eq. I we can solve for x:
x+y=24 Eq. I
x+(2x-24)=24 substitute value for y from Eq. II
Therefore, Ellen's present age is 16.