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Five Sentences (Posted on 2004-10-12) Difficulty: 3 of 5
Which of the following 5 sentences are true?

a) It is not the case that 2 consecutive sentences are both false.
b) There are fewer false than true sentences.
c) It is not the case that 3 consecutive sentences are all false.
d) It is not the case that 2 consecutive sentences are both true.
e) There are exactly 3 false sentences.

See The Solution Submitted by Brian Smith    
Rating: 3.2308 (13 votes)

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Solution | Comment 16 of 34 |
I believe the solution can be either FFFTF

To arrive at this conclusion I first determined if a. could be true using the following logic:

If a. is true then b. must be false using the following reasoning:
b. as true would make d. false since it prohibits consecutive trues and e. false since it requires more false than true.  d. and e. would then become consecutive false not allowed by a. being true.
Thus if a. is true b. must be false

If b. is false then at least two of the next three sentences must be false which is not possible for a. to remain true. 

Thus, a. cannot possibly be true and is therefore false

I next  set about to determine if b. could be true if a. was false using the following logic:

If b. is true then e. must be false which would fill the maximum 2 false statements ( a. and e.) required by b. and would require c. and d. to be true.   This is not possible considering that d. prohibits two consecutive true sentences.

Thus, both a. and b. are false

I then determined that c. must also be false using the following logic:

If c. is true then d. becomes a self contradictory statement  as follows:
If d. is true then c. and d. form consecutive true statements making it false.
If d. is false then there is no place for consecutive true statements to exist.

Therefore a., b., and c. are false

d. must then be a true statement since makeing it false would contradict itself.

Since d. is true e. must then be false (which also creates the fourth false statement)


  Posted by Bstratford on 2004-10-19 19:26:35
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