Raymond Smullyan has a character pose the following question (changed it so that it makes more sense in this context):
A certain person (to whom I shall refer to by the letter A) who must be a knight (a person who always tells the truth) or a knave (a person who always lies) makes a certain statement to another person (whom I shall call S for convenience). S did not know whether the statement was true or false before the statement was made and did not know whether A was a knight or knave before the statement was made. However, after A makes this statement to S, S is able to conclude that the statement is true and A is a knight. What statement does A make?
Smullyan gives the following answer: "I am not a married knight."
My answer (before seeing Smullyan's) was: "I'm a married knight or I'm a a knave".
My question is whether my answer works, too.
Your answer works for sure, but there's a problem in the way it works. If you look carefully, your answer is of the form $A\lor B$. So, you are basically making two different statements masked as one. On the other hand, the answer that your book gives makes only one statement. So, your solution is weaker that the one in your book. And often, in these type of problems, we prefer not to have $A\lor B$ type statements (since that really doesn't keep the essence of the statement alive), especially when it is possible to avoid them. Notice that you can make as many statements as you want if you decide to use the "or" connector. So, things would no longer remain interesting if you're allowed to use them.
Do you get my point?