Which kangaroos should we trust?

Logic Level 2

There are at least two kangaroos in the enclosure.

One of them said: “There are 6 of us here” and he jumped out of the enclosure.

Afterwards, every minute one kangaroo was jumping out of the enclosure saying: “Everybody who jumped out before me was lying.”

This continued until there were no kangaroos left in the enclosure.

How many kangaroos were telling the truth?

Source: MathFights .
4 3 5 0 7 1 6 2

Prasun Biswas by Prasun Biswas , 23, India

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2 solutions

Harish Sasikumar
Nov 16, 2015

It can only be one of the 2 cases (the first kangaroo can tell the truth or lie)

Case 1: first kangaroo was telling the truth

All the subsequent kangaroos are lying. Hence the number of true statements is 1

Case 2: first kangaroo was lying.

The immediate next kangaroo's statement will be true. All the subsequent kangaroos will be lying. Hence the number of true statements is 1

3 Helpful 0 Interesting 0 Brilliant 0 Confused
Colin Carmody
Nov 16, 2015

If you assume the first kangaroo's statement to be true, then all of the other kangaroo's would be accusing him of lying, but we said he was not, so therefore he is the only one telling the truth.

If you assume he is lying, it is complicated and you can't figure out who is lying with the information given. So therefore, the answer is 1.

0 Helpful 0 Interesting 0 Brilliant 0 Confused

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