A careful breakdown follows.
Nine sons form the first group.
One sister forms the sibling shared among the sons.
One father forms the parent of the children.
The total becomes nine plus one plus one. The sum equals eleven people.
The solution emerges through close attention to the given words.
Many individuals reach an incorrect total because of the way the sentence is constructed. The phrase “each son has a sister” leads the mind toward an image of multiple distinct sisters. The assumption of uniqueness appears automatically.
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