When Ben was on math team as a sophomore, his team decided that when you had to take a guess, the answer was 7. It was right more often than you'd think, and they even considered shirts emblazoned with it.
That "7" would occur with extra frequency was not a surprise to me once I considered the matter. I suggested that -1 would also be common, and at upper levels, 2pi revolutions would show up more often than you'd expect. Seven is just beyond the level where you might get lucky and notice a relationship. If you saw a 216 in a long numerator and a 108 in the denominator, a clever person might pick up the connection and save computational steps. Anything ending in 5 or 0 is going to have obvious factoring. But seven, no one is going to see into a seven - relationship by luck.
These math test makers are essentially evangelists. They want you to see the elegant beauty of how complicated, messy conglomerations resolve to something simple. 29 is also beyond intuitive math, as is 9.7, but those aren't so much fun. You have to think like these math teachers - they think it is just very cool when numbers operate cleanly like that. But they don't want to make it too easy. Thus, the use of the answer -1, instead of just 1, which would of course be the ultimate in cool resolutions. With -1 you have to keep track of the signs. Heheheh, a lot of them won't be careful at this point and they'll forget to switch the sign. Won't they feel silly, then?
The SAT and other standardised test people don't design things this way. They might want to, but they have to make you grind out the answers, so that 13.2 is a real possibility. They're just trying to test you accurately, not convince you of math's elegance.
You have to think like the people making the test. As the remarkable book Up Your SAT (the precursor to Up Your Score) puts it, the paragraph in the comprehension section is not going to say that Beethoven sucks. That will never be one of the main ideas you're supposed to carry down to the multiple-choice questions below. Being able to think like a test-maker is worth an unfair number of points. Of course, one could argue that figuring out how a test-maker thinks is itself a measure of intelligence.
Math team, when in doubt, guess 7, or -1 if they're playing with the signs. There might be some bias toward those answers on the SAT, but nowhere near as strong.