Hmm, you are right. According to the constraint from the question, y must be divisible by 6, so y must be even, and xy must be even. GMAT questions are designed to always require more info than that given in the question. I'll ask one of the GMAT Club gurus to look into this.
(Side note: you said "y = 0 or any multiple of 6," but that's redundant. Zero actually is a multiple of 6 itself, because 0/6 = 0= integer.)
Emily Sledge | Manhattan GMAT Instructor | St. Louis
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