Ninety percent or more of people in general have average IQs. Supposing that the smart people are equally offset by the dumb people, and factoring that Mensa regard the upper two percent as 'smart' [although the upper two percent still allows for subgeniuses], we could assume that ninety-six percent are average [C], two percent are smart [B and A], and two percent are clinically retarded [D and F].
     Of course, that's a serious oversimplification, but it kinda addresses the nature of the question. The fact of the matter is that, while four questions answered correctly out of five is average, twenty-four people out of twenty-five will get only those four questions right.
--Gremlin

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