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

2y-y=2x-24

y=2x-24

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

3x=24+24

3x=48

x=16

Therefore, Ellen's present age is 16.