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Girls and boys

Philippe_57721 · Math ·

At school, little boys are mischievous: they pull girls' hair, they kick. Girls don't like boys and prefer to be with girls.
In a classromm a teacher decides to place children in row in such a way that each girl gives the hand to at least one other girl.
B G G B - good
B G B G - not good

How many possibilities are there in a classroom with 40 children