remember!
the OG answer keys are usually incomplete—and quite often incorrect.
the official PROBLEMS are essentially flawless.
the official ANSWER KEYS, on the other hand, most certainly are not.
GMAC's 'all-stars'—the people who create the problems—do not write the answer keys.
instead, GMAC 'outsources' that work to people of lesser talent, presumably to cut costs.
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interesting, by the way, that this problem is in OG. there aren't very many problems that appear in both sources.