Well, I'm new here, and although I'm still about a year away from the MCAT's and haven't begun to prepare, I'd like to think I'm intelligent enough to see that those two statements are in no way equivalent without contextual clarification. 😛 In other words, I'm with liverotcod. 🙂
In statement A, we are dealing with a percentage of people; in statement B, we are dealing with a percentage of lies believed. Thus, they are inherently saying two different things. The only way I could see to easily rectify this would be if it were assumed that each untrained person in a sample was to hear only a single statement (thus obviating the "75% of lies" conundrum) which could be interpreted as either a lie or the truth; thus, it is distilled down to a binary choice T or F. Now, if untrained persons are held as believing 75% of the lies they are told, then 75% of whatever size sample of untrained persons is queried would necessarily have to believe a false statement to be true (since True or False doesn't allow for percentages within a single person's answer), and thus will be counted under the set of those found in statement A (i.e., this results in 75% of untrained people believing the lies they are told, as expressed in statement A).
But this is just a hypothetical scenario which would serve to "prove" the two to be equivalent under stringent conditions ; under normal conditions, when standing alone, I do not believe them to be so, though I could be mistaken-- formal logic was never my forte. 😛 I also believe that the above rationale (if anyone can make sense of that mess 😀) also holds in converse, though it's much too late for me to consider it. 🙂 As far as the abstract logical concept of "equivalence" goes, however, you'd still only be instantiating a single instance with very specific conditions using my above reasoning, and so I don't believe that it would hold up under a broader interpretation (e.g., are these equivalent in general under all circumstances?) barring clarification.
I'd be interested in hearing if A) anyone understands what I'm saying above and feel it applies to this problem (because it was initially vexing for me as well), and B) the correct answer to the problem, along with the accompanying explanation from the text, from the original poster. 🙂
And yes, I think too much. 😀