Final answer:
Based on the given information, Helen is 26 years old.
Step-by-step explanation:
Let's represent Helen's age as x.
According to the given information, John is twice as old as Helen, so John's age can be represented as 2x.
Subtracting 4 from John's age and then dividing by 2 gives William's age. So, William's age is (2x - 4)/2 = x - 2.
Given that William's age is 24, we can set up the equation x - 2 = 24 and solve for x.
Adding 2 to both sides of the equation, we get x = 26.
Therefore, Helen is 26 years old.