Puzzles
Every sister has as many brothers as sisters; every brother has double that in sisters
In one family, each daughter has exactly as many brothers as she has sisters, and each son has exactly twice as many sisters as he has brothers. How many sons and daughters are in the family?
Reveal the answer
4 daughters and 3 sons. Each girl counts 3 other sisters and needs an equal 3 brothers; each boy counts 4 sisters and needs half that many other brothers, 2 — solving those two simple equations together gives a unique answer. It's one of the family-relationship puzzles collected by Soviet mathematician Boris Kordemsky in 'The Moscow Puzzles' (1956), translated into English only in 1972.
— Boris Kordemsky, The Moscow Puzzles: 359 Mathematical Recreations — 1956; English translation 1972
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