Code:
 5    2
14   20
26    ?
Normally I don't bother with problems like this one because, as is well known, you can justify any answer whatsoever. But the source of this problem makes it intriguing. (No, I don't know the answer.)

The problem originally appeared in Dr. Dobb's, a programming journal, in an ad from Google Labs a few years ago. Google Labs published several problems around that time, with the implied promise (probably false) that if you could solve one or more then your resume would receive serious consideration if you applied for a job. Some of these problems required some heavy-duty math and/or programming skills, such as searching for prime numbers in the digits of pi. See, for example,

MathWorld News: Mathematica's Google Aptitude

The original ad consisted of a photograph of an empty parking garage with the numbers painted on pillars. The viewpoint was from the center front of the 3 x 2 array, so another possible interpretation of the problem, reading the numbers from left to right, is

Code:
26  14  5  2  20  ?
.

Any ideas?