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The answer is YES. But if d=0.1, than 12d=1.2 and it is not divisible by 6. Any thoughts?
You are right if we were not told that d is an integer then 12d may or may not be divisible by 6, consider d=1 and d=1/3. Though MGMAT states on that page that "all variables in problems #1 through #11 are assumed to be integers unless otherwise indicated." Hence their answer YES is correct.
Also notice that every GMAT divisibility question will tell you in advance that any unknowns represent positive integers (ALL GMAT divisibility questions are limited to positive integers only).