From Litton's Problematical Recreations
It is tempting to argue as follows:
At most one statement can be true (they are mutually contradictory). If they are all false, statement 100 would be true, which is no good. So 99 are false, and statement 99 is true.
If replaced by "at least", and the "real" number of false statements is x, then statements x+1 to 100 will be false (since they falsely claim that there are more false statements than there actually are). So, 100-x are false, ie. x=100-x, so x=50. The first 50 statements are true, and statements 51 to 100 are false.
However, there is a hidden and incorrect assumption in this argument. To see this, suppose that there is one statement on the sheet and it says "One statement is false" or "At least one statement is false," either way it implies "this statement is false," which is a familiar paradoxical statement. We have learned that this paradox arises because of the false assumption that all statements are either true or false. This is the hidden assumption in the above reasoning.
- Another example of the fallacy of this assumption is the following. Suppose there are two statements in a box
- 1. Both statements in this box are false. 2. The egg is in box #5
Now, obviously statement 1 cannot be true, since this would lead to a contradiction. But if statement 1 is false, and statemnet 2 is false also, then again there is a contradiction. The only internally consistent conclusion is that statement 2 is true. But of course that doesn't mean that statement 2 is true, since statement 2 could be anything.
If it is acknowledged that some of the statements on the page may be neither true nor false (i.e., meaningless), then nothing whatsoever can be concluded about which statements are true or false.
- complicated version of the same puzzle is
1) At least one of the last two statements in this list is true.
3) There exist three consecutive false statements.
6) This is not the last true statement.
7) Each true statement's number is a factor of the unknown number.
10) There are no three consecutive true statements.
What is the number?
The incorrect but plausible solution is:
By 2, either way 1 must be false, and then so must both 9 and 10.
6, if false, says "This is the last true statement", which gives a paradox, thus 6 must be true, and so must 7 and/or 8.
7 and 8 cannot both be true, as the number had to be a multiple of 6,7,8 , that is a multiple of 168 (by 7), and less than 100 (by 8)
If 8 is false, then 3 is true (8,9,10 is false), if 8 is true, then 3 is false (3 cannot be true when both 6 and 8 are true, then there are no three consecutive statements left).
So we have either
A) F X F X X T F T F F or B) F X T X X T T F F F
In A), 4 and 5 must be true (by false 10), and 2 may be true or false. So by 5 the number shall be either 27 (2+3+4+5+6+7) or 25 (3+4+5+6+7). None of these can fullfill 8, though, so A) is out leaving us with B)
Now, (by 7) 3,6 and 7 are factors, the number must be a multiple of 42, as 2+3+4+5+6+7=27, 5 must be false.