Ajay is a friend of Rakesh. Pointing to an old man, Ajay asked Rakesh who he…

Concept

A blood-relation puzzle is solved by translating each kinship word into the generation-and-sibling relation that defines it, and then resolving the chain one link at a time starting from the speaker. A paternal uncle is, by definition, the brother of a person’s father, and the Hindi term chacha used in this item carries exactly that restriction. When a single statement describes one man in two different ways, as "A’s son" and as "B’s son’s paternal uncle", identifying that one man is what fixes A’s position relative to B.

Applying it here

1. The speaker is Rakesh, so "my son" is Rakesh’s son and "his son" is the old man’s son.

2. "My son’s paternal uncle" means the brother of the father of Rakesh’s son. Rakesh is his own son’s father, so his son’s paternal uncle is Rakesh’s own brother.

3. The statement equates the two descriptions of the same man, so the old man’s son is Rakesh’s brother.

4. A man whose son is Rakesh’s brother is the father of Rakesh’s brother, and the father of Rakesh’s brother is also the father of Rakesh.

5. Hence the old man stands to Rakesh as his father.

Contrasting the four relations

Result

The statement fixes the old man’s son as Rakesh’s brother, so the old man is Rakesh’s father.