Who's the lady?

Logic Level 2

Pointing at a photo, a boy named Tom said, "She's the daughter of my paternal grandfather's only daughter-in-law".

If the only people that have children in this family tree are married, then how is the girl related to Tom?

Cousin Sister Aunt Daughter Sister

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Skand Singh by Skand Singh , 28, India

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8 solutions

Brad Thompson
Nov 8, 2015

This all assumes you can only have kids if you're married.

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I actually agree with this and think this is possibly a bad problem. It relies on a variety of assumptions of gender (maybe Tom was adopted by his paternal grandfather's gay son) and marriage. Even in a traditional marriage structure children born out of wedlock are a possibility. Ok so father's father's only daughter in law implies the woman must be his mother because of the all male relations, I think I still would have missed this but tom's father might have had Tom out of Wedlock and then Tom's father's sister might be in a same sex marriage and then impregnated herself from a sperm bank. There are numerous possibilities to get around the implied relations in this problem. It's a nice logic puzzle, but the metaphor is invalid. Anyways I need to spend more time pondering these and less rushing in, but there a various valid family relationships that could make the girl his cousin.

Andrew Jones - 5 years, 7 months ago

Wao, great thinking man. So, tell me the answer with your point of view.

Skand Singh - 5 years, 7 months ago

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I was thinking the same as Brad. Maybe Tom's paternal grandfather had two sons. Tom's father never married but his brother did. That would make the lady in the painting his cousin. Which is why I answered Cousin Sister.

Glen Mast - 5 years, 7 months ago

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100% correct Really stupid answer!

It could be cousin or sister!

Kareem Mohamed - 5 years, 7 months ago
Skand Singh
Nov 1, 2015

Only daughter-in-law of Tom's paternal grandfather will be his mother and the girl in picture is daughter of that lady, which means, she's sister of Tom

7 Helpful 0 Interesting 0 Brilliant 0 Confused

Why couldn't tom's father be unmarried?

Paul Alberti-Strait - 5 years, 7 months ago

Why can't Tom be his paternal grandfather's biological daughters son? That would then make the lady his cousin or am I missing something. It doesn't state that his grandfather had no daughters only that he had one daughter in law

Rob Brown - 5 years, 7 months ago

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Paternal grandfather, not maternal.

Umar Khawaja - 5 years, 7 months ago

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Thank you, Learn something new every day, I just presumed "paternal" grandfather meant male, I didnt realise it meant fathers father, and maternal grandfather meant mothers father

Robert Brown - 5 years, 7 months ago

You asked "Why can't Tom be his paternal grandfather's biological daughters son?" If Tom will be his paternal grandfather's biological daughter's son then the name "paternal" will be changed to "maternal" then his grandfather will be no more "paternal" rather he'll be "maternal". Let me take you through this example. Tom's paternal grandfather means Tom's father's father, and when you say "daughter's son" this means the grandfather is maternal and not paternal. Be focused.

Skand Singh - 5 years, 7 months ago

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Thank you, Learn something new every day, I just presumed "paternal" grandfather meant male, I didnt realise it meant fathers father, and maternal grandfather meant mothers father

Robert Brown - 5 years, 7 months ago

There is another reason:the problem states his only daughter in law, which indicates that no other woman married into the family, which means, by default, the only daughter in law to his paternal grandfather IS Tom's mother

John King - 5 years, 7 months ago

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@John King – But how do you know that Tom's mother is married to Tom's father?

Paul Alberti-Strait - 5 years, 7 months ago

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@Paul Alberti-Strait – Like I said earlier, always assume the simplest scenario when solving problems like these, even if a real life situation presents multiple valid options

John King - 5 years, 7 months ago

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@John King – Why is it simpler to assume that one has to be married in order to have children? It seems simpler to not assume that requirement. It would be pretty simple to add a condition to the question like "Neither Tom nor the girl were born to unmarried parents." Or plenty of other ways the question could have been phrased. As written, the question is just a bad question. There is no defending it.

Paul Alberti-Strait - 5 years, 7 months ago

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@Paul Alberti-Strait – in a real life scenario, I agree with you. however logic problems should be approached from a mathematically logical point of view, not necessary a common sense view

John King - 5 years, 7 months ago

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@John King – I'm still not following why the assumption that all children are born of married parents is mathematically logical, or why this question should be defended as written when it could be so easily fixed.

Paul Alberti-Strait - 5 years, 7 months ago

@Paul Alberti-Strait – Again, like I keep saying over and over, with logic problems, you have to look at them from a pure mathematical sense, not exactly a common sense one

John King - 5 years, 7 months ago

@John King – Unless Tom was conceived out of wedlock, or if Tom's mother conceived Tom, later underwent gender reassignment surgery, and stayed with the same man (or any man, really).

The latter example is, of course, much less likely and quite contrived, but it's still a valid option.

Liam MacTurk - 5 years, 7 months ago

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@Liam MacTurk – I get that. However with problems like these, it's best to think of them in the simplest form, leaving out things that would be considered in a real life scenario and not one based on pure logic

John King - 5 years, 7 months ago

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@John King – Yeah, of course. Unless otherwise stated, one should typically assume the question is referring to the most commonly understood definitions and situations (unless, of course, you already know the poster is someone who says exactly what they mean, and means exactly what they say). It's been somewhat frustrating getting acclimated to Brilliant for me, at least, because I'm one of those people who uses exactingly precise language, and have always expected textbooks, literature, and learning/practicing resources like these to do the same... but here, it seems the vast majority of the problems include lots of unstated assumptions, implied subtext, etc. :)

Liam MacTurk - 5 years, 7 months ago
Abhijeet Verma
Nov 1, 2015

Well, Tom needs to be a boy,or else the answer "Self" needs to be included as well...!

6 Helpful 0 Interesting 0 Brilliant 0 Confused

Thanks folk, I corrected it.

Skand Singh - 5 years, 7 months ago
Veritas De Vita
Nov 11, 2015

It could be that she is his Cousin Sister:

Suppose Tom's paternal grandfather has two children, both boys. The first boy, Tom's father, never married, and Tom is his son. The second has married a woman, and they have a daughter. This daughter is the girl referred to in the question. (Edit: Just saw this same answer posted above.)

As an extra point, alternative solutions are available if Tom's grandfather's children have same-sex marriages/relationships.

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Ben Tennyson
Nov 11, 2015

Paternal grandfather means Tom's Father's father,his only daughter in law means his father's wife. The daughter of his father's wife means his sister....... He's got a very beautiful sister..XP

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Katia Leiva
Dec 5, 2015

The answer is sister because of this family tree has to be married to have children then Tom's parents must be married considering they had him. The next part is that the girl in the painting is his dad's dad's ONLY daughter-in-law's daughter, which is stating that he only has one child whom is married, making it Tom's parents.

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Eric Christianson
Nov 11, 2015

I agree it could be sister or cousin but cousin sister is not related to the boy. His cousin (uncles son/daughter) sister would allso be a cousin unless hes is hafe sister to the cousin and no relation to the boy.

So cant be cousin sister Cant be daughter or aunt clearly That leaves sister

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Cousin sister is not used to indicate a sister to the cousin (that would be the Cousin's sister). It is used to specify the gender of the cousin in question (in the case of cousin sister, it is indicating a female cousin).

Steven Harbin - 5 years, 7 months ago

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Tomato tomåto

I have never heard cousin sister before it has allways been referred to as female cousin.

Eric Christianson - 5 years, 7 months ago
Prithvi Joshi
Nov 11, 2015

Only daughter in law of grandfather would be his mother itself. So the daughter of his mother is his sister

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