A man walks up to a house and knocks on the door.
He hears a woman asking: "Who is it?"
The man answers: "My mother-in-law and your mother-in-law are mother and daughter"
What is the relationship between the man and the woman?
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The man can be the girl's father-in-law.
The man can also be the girl's mother-in-law's brother-in-law. (try saying this 3 times out loud)
Let's make this notations:
M - the man
G - the girl
MML - man's mother-in-law
GML - girl's mother-in-law.
We get this:
mother- daughter/ \ mother-in-law - son-in-law
From this tree we can deduce that
M is the husband of GML hence his is the girl's father-in-law.
Or GML has a sister or brother and M is married to that brother or sister.
The woman could also be
The man's mother-in-law, because she is married to his spouse's mother
Man is father-in-Law of the girl
If Daughter's mother-in-law is man's wife, so that man's mother-in-law is his wife's Mother. So in relation Man is father-in-law of the girl.
The key statement is, “My mother-in-law and your mother-in-law are mother and daughter.” Note that this statement omits the critical word, respectively, so the order of the two lists is not necessarily the same. The accepted answer assumes that the mother-in-law of the man (who knocked) is the mother of the mother-in-law of the girl/woman/female who was inside the house. But it’s possible, as in Dane Andersen’s answer, that the mother-in-law of the man (who knocked) is the daughter of the mother-in-law of the girl/woman/female who was inside the house. This leads us to the following family tree (where “=” represents marriage, and “/”, “|” and “\” represent parent-child relationships). Let M0 and F0 represent the man and the female who speak in the riddle (respectively).
F3 / \ F2 M4 = F0 | M0 = F1
i.e., the man could be
the son-in-law of the woman’s sister-in-law.
The widower married a woman whose widowed mother married the widower's son.
I see other answers are very intricate, but in the fact the answer is simple:
If I use this notations:
M - the man G - the girl
G is M Aunt.
I mean G is M.Mother sister.
G.mother and M.mother are mother and daughter.
I'm confused with accepted answer really !