# A question regarding "Who is born earlier in time"

1. Jan 24, 2016

### simplex1

http://math.stackexchange.com/questions/1625487/who-is-born-earlier-in-time-the-great-grandson-of-the-grandson-or-the-grandson [Broken]

Last edited by a moderator: May 7, 2017
2. Jan 24, 2016

### phinds

How would you go about figuring this out (hint: it's trivial)

3. Jan 24, 2016

### DaveC426913

Actually, there is insufficient information. There's no way to tell which one was born earlier.

I sure hope this isn't a homework question...

4. Jan 24, 2016

### simplex1

I have found the question here: http://math.stackexchange.com/questions/1625487/who-is-born-earlier-in-time-the-great-grandson-of-the-grandson-or-the-grandson [Broken] a place where the moderators considered it ill-defined, quote: "You need to study the theory of definite descriptions to help you avoid posing ill-defined questions."

In my opinion the problem has at least a nice solution.

Last edited by a moderator: May 7, 2017
5. Jan 24, 2016

### DaveC426913

Yup. It is insufficiently defined to provide an answer.

phinds has egg on his face. He thought the answer was trivial. but right about now, he's realizing he went off half-cocked.

6. Jan 24, 2016

### simplex1

Just propose an answer, a solution, for a particular case of your choice.

7. Jan 24, 2016

### DaveC426913

Since the question cannot be answered as it stands, how is one supposed to answer it?

The only correct answer is: I don't know, and neither do you.

8. Jan 24, 2016

### simplex1

Take the case "one child policy".

9. Jan 24, 2016

### DaveC426913

There you go! That must be specified in the question.

And now the question is trivial.

10. Jan 24, 2016

### phinds

No, I made a simplifying assumption that the ages at which everyone had their so was the same. That makes it trivial. The point is to simply count generations. A better-formed question would be "how many generations is that" and that is trivial.

11. Jan 24, 2016

### phinds

No, not YOU have egg on your face, at least to the same extent that I did. You have made the same simplifying assumption I did.

12. Jan 24, 2016

### DaveC426913

Why have you
a] made an unfounded assumption, and
b] completely reformulated the question,
c] without stating your case-reducing assumption in the answer (necessary to make it correct)?

Last edited: Jan 24, 2016
13. Jan 24, 2016

### DaveC426913

No I didn't.

The OP provided the missing criterion in post 8. 'One child policy' means that the answer is so trivial it is degenerate (Which is younger: Person A or the same person?) I'm not sure why you think I made any simplifying assumptions.

14. Jan 24, 2016

### phinds

But the one child criterion does not make for a unique answer. You have to make the simplifying assumption of same age births. Think it through, you'll get it. Assume all births are after a different number of years. Clearly you don't end up w/ the same answer (necessarily).

15. Jan 24, 2016

### DaveC426913

Yes it does.

You are comparing the age of a person to themselves. They are the same person.

I think you might have overlooked the last criterion in the OP: the ancestor is the same person in both scenarios.

16. Jan 24, 2016

### phinds

You're missing the point. Write it out, using different ages for each person when their son is born. You get different answers and which path leads to a younger person will vary depending on those ages.

17. Jan 24, 2016

### DaveC426913

See post 15.

18. Jan 24, 2016

### phinds

Yeah, you're right. I goofed. Definitely egg on my face, not yours. Thanks.

19. Jan 24, 2016

### DaveC426913

It's a really tricky one!

20. Jan 24, 2016

### phinds

Doesn't seem all that tricky if you think it through properly, which I did not at first.