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?
Puzzling Stack Exchange is a question and answer site for those who create, solve, and study puzzles. It only takes a minute to sign up.Sign up to join this community
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 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.