FF, what precisely was the error?
The error is, as Faldage pointed out, with the use of AND in the first sentence. It should be an OR. In casual conversation, one can use words as one pleases. The criterion of correctness is whether the recipient of the sentence understands it. But in logic problems, some words have very specific meanings, AND being one of them. If you say "P and Q," that means both.
Now we could argue that the student should know what was meant. It then becomes a problem of linguistic interpretation and not a problem about about logical reasoning.
(Back when I took AI, one of the more challenging exercises we had was to take a list of sentences in common English, convert them to statements in the predicate calculus, then reword them to say what the author really meant and convert THOSE to predicate calculus. I know it sounds trivial, but it took a lot of effort to do this.)
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