If we say that the weight of Jill's cat is w, then we can say that the weight of Jack's cat is 4w.
We know that the combined weight is 35 pounds, so we can write this as
4w+w=35
Which simplifies to
5w=35
If we divide by 5, we are left with
w=7
So that gives us the weight of Jill's cat. If Jack's cat weighs 4 times as much, we can see that
4x7=28
Therefore, Jack's cat weighs 28 pounds :)
