Good girls and bad boys (2)

Logic Level 1

On an island, there is this strange pattern - boys always lie while girls always tell the truth. You encounter two children.

  • The older one says: "I have a younger brother."

  • The younger one says: "We are of the same gender."

Who is speaking to you?

Two sisters Older brother, younger sister Older sister, younger brother Two brothers

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Noel Lo by Noel Lo , 24, Singapore

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2 solutions

Noel Lo
Jan 26, 2016

Suppose the older one is a boy. Then the statement "I have a younger brother" is false since he is a liar. This means he has a younger sister. Suppose, on the other hand, the older one is a girl. Then since she is a truth teller, she indeed has a younger brother.

We cannot really pinpoint which case is true but either way, the two children are of opposite genders. This makes the younger sibling's statement false. So the younger sibling is indeed a boy since boys lie. The older sibling's statement is therefore true so she must be a girl.

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Saya Suka
Mar 6, 2021

Whenever somebody claimed to be associated with another person / party in a bigger group, then we can be sure that the other person is a truth teller even when that someone who did the whole self-claiming business might do so deceptively, too. From this, we know that the older sister is telling the truth with her "I have a younger brother" speech. They must be a pair of older sister and younger brother.

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