<p>I remember this question. The options were:</p>
<p>I. 6
II. 6.5
III. 7</p>
<p>And the letter choices were some combination of these that I don't remember.</p>
<p>But anyway, the key was that you could only add one mystery number to the right or left side of a collection of an even number of numbers. Somehow, because after adding one more number, the collection had an odd number of numbers, the answer could only be an integer.</p>