In such type of questions, clues are given regarding comparisons among a set of persons or things with respect to one or more qualities. The candidate is required to analyse the whole information, form a proper ascending / descending sequence and then answer the given questions accordingly.
B is twice as old as A but twice younger than F.
C is half the age of A but twice the age of D.
Which two persons from the pair of oldest and youngest ?
Let A's age be x.
Then, B's age is 2x.
B is twice younger than F i.e. F is twice older than B. So, F's age is 4x.
C is half the age of A i.e. C's age is x/2. C is twice the age of D i.e D is half the age of C i.e. D's age is x/4.
So, the descending order of ages is F, B, A, C, D/
Clearly, F is the oldest and D is the youngest.