A girl introduced a boy as the son of the daughter of the father of her uncle. Who is the boy to the girl?

Introduction / Context:
This blood relation question uses a long chain: the son of the daughter of the father of her uncle. You must break this phrase into smaller parts such as uncle, uncle's father and that father's daughter, and then see which familiar relatives they correspond to. Finally, you compare the boy's position in the family with the girl's position to work out whether he is her brother, cousin, nephew or something else.

Given Data / Assumptions:
- Reference person: a girl (call her G). - She has an uncle. - We interpret her uncle as her father's brother (paternal uncle), which is the standard assumption in exam questions unless otherwise specified. - The father of her uncle is then her grandfather on her father's side. - The daughter of that grandfather is one of his daughters, typically G's paternal aunt. - The boy is the son of that daughter.

Concept / Approach:
Start from the phrase her uncle and move stepwise upwards and then downwards in the family tree. Her uncle's father is her grandfather. A daughter of that grandfather is a sister of G's father, that is, G's paternal aunt. The boy is then the son of G's paternal aunt. Children of aunts and uncles are cousins to the girl. So we expect the boy to be her cousin. The challenge is to resist the temptation to treat the daughter as G's mother, which would be wrong here.

Step-by-Step Solution:
Step 1: Her uncle is taken as her father's brother (paternal uncle). Step 2: The father of her uncle is the father of her father as well, so he is G's paternal grandfather. Step 3: The daughter of the father of her uncle is a daughter of G's grandfather. That daughter is G's paternal aunt (her father's sister). Step 4: The boy is the son of that daughter, which means he is the son of G's paternal aunt. Step 5: A son of an aunt is a cousin to G. Therefore, the boy is G's cousin.

Verification / Alternative check:
Draw a small family diagram: at the top, the grandfather; one level below, his children – G's father, G's uncle (father's brother) and G's aunt (father's sister). On the next level, you place G as the daughter of the father, and the boy as the son of the aunt. Comparing their positions, both are grandchildren of the same grandfather but through different parents, which is exactly the definition of cousins.

Why Other Options Are Wrong:
- Son: For the boy to be G's son, G would have to be from a generation above the boy, which is not supported here. - Brother: A brother would share both parents with G. Here, the boy's mother is G's aunt, not G's mother. - Nephew: A nephew would be the son of G's sibling, but the boy is the son of G's aunt, not of her sibling. - Uncle: An uncle is a brother of a parent or the husband of an aunt, not a child of an aunt.

Common Pitfalls:
One common confusion is to interpret father of her uncle as father of her father and then daughter of that father as her mother. However, the phrase clearly indicates a daughter of the grandfather, which, on the paternal side, is an aunt rather than the mother (who is generally not described as daughter of paternal grandfather in these puzzles). Keeping generations and typical exam assumptions clear helps avoid this trap.

Final Answer:
The boy is the girl's cousin.