Good girls and bad boys (1)

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: "We are of the same gender."

  • The younger one says: "We are of opposite genders."

Who is speaking to you?

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

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2 solutions

Noel Lo
Jan 26, 2016

The two statements contradict each other. Obviously, one must be true, the other must be false. Since we know boys lie while girls tell the truth, one must be a girl, the other a boy. We now can tell that the two are of opposite genders so the older one is a liar (and hence a boy) while the younger one is a truth teller (and hence a girl).

But how you can determine who's the older one and the younger one?

Rahmat Hidayat - 5 years, 4 months ago

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It is mentioned in the question.

Noel Lo - 5 years, 4 months ago

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no, i mean how you can determine the older one is a boy, and the younger one is a girl?

Rahmat Hidayat - 5 years, 4 months ago

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@Rahmat Hidayat Because one says same gender, the other says opposite gender. Whichever way possible, one of these claims is correct, the other wrong. So this means one truth and one lie. Since boys always lie, girls always tell the truth, there must be one boy and one girl. This gives us opposite genders. So the one who said same gender is a liar (a boy) while the one who said opposite gender is correct (a girl).

Noel Lo - 5 years, 4 months ago
Saya Suka
Mar 6, 2021

Those two are saying two polar opposite things, so they must be of different genders as stated by the younger sibling. Thus, we have here an older brother and a younger sister.

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