<p>They were consecutive multiples of three.</p>
<p>The last question, I narrowed it down to 5 and one other choice, because any multiple of three has to have all its digits add up to a multiple of three. At this point I would have guessed, but I had some extra time so I performed manual calculations of consecutive multiples of three to find the right answer.</p>
<p>I spent several minutes on that one, I think, just plugging in numbers systematically.</p>