Which number?

Dec 2019
52
1
ok
If Beta had 3, Gamma's number could be 1, 2, 5, 7 or 8, which is 5 possibilities, not 3.

If Beta had 7, Gamma's number could be 1, 2 or 3, which is 3 possibilities. and couldn't exceed 3, as that would require Alpha's number to be less than 4, which is already ruled out.

In a similar way, it turns out that Beta's number is 4, 6, 7 or 9.
Ooooh I get it now. But how can you determine Alpha's number with those possibilities?
 

skipjack

Forum Staff
Dec 2006
21,321
2,386
Given that Alpha's number is 4, 5, 8 or 10, and Beta's number is 4, 6, 7 or 9, there are 16 possibilities, of which only four allow Gamma to deduce Alpha's number, namely when Gamma's number is 5, 6, 9 or 10. The two cases where Gamma's number is 5 or 10 are interesting, because they both allow Gamma to know that Alpha's number is 4, but Alpha doesn't have enough information to reduce the shortlist to just one case. As soon as Alpha reveals that situation, Beta can deduce what Alpha's number is, but we don't need to know Beta's exclamation about that.

We already know that Alpha's number is 4. Beta's number may be 9 (in which case, Gamma's number is 5) or 6 (in which case, Gamma's number is 10).