I didn't know it was timed, though I suspected it. Regardless, I had to alt-tab away from the test a couple of times for work-related reasons. If I'd seen the timer, I would have instead cancelled the test and done it when I got home, when I'd have no distractions. Just something to consider... I don't know whether that's something you'd want to control for or not, it depends on what you're aiming to measure. But might be worth discussing in your results regardless... "they thought they had time to think about the answer" isn't the only reason they might take longer. And it means the results of this won't be directly comparable to a similar experiment done IRL, which would presumably be comparing distractionless-environment vs distractionless-environment-plus-timer.
The term "opposite number" for the arithmetic inverse isn't one I'd come across before... I'd always heard it as "negation". I suspected from context that that may have been what it meant (given the options about positive and negative numbers in the answers) but I wasn't sure, so I hit "Skip" to be fair. And then checked Wikipedia to confirm that my guess was correct (while still on the clock).
Also, I totally screwed up the 3x+6y=2 question, by misreading it as x+6y=2, and I thought that all of the answers were wrong... and I had a whole line in here about that, but then I realised my mistake (I thought it was a typo, because one of the answers would have been correct if the question was instead x-6y=2). I answered "Skip" to it, though, so I got to the end with no wrong answers (my score was 11-0).