Thanks to Lloyd for explaining basic number theory to me on IRC - I can now see how I should have answered the exam question on Tuesday :)
One odd thing yesterday was that Bubbles (cat) went AWOL. I let her out in the morning when I went to my exam, but there was no sign of her when I got back. I wandered round the estate looking for her, but she didn't show up by the time I went to bed. No sign of her this morning either, and I was really starting to worry about the attrition rates of cats in our house. But she's now returned, safe and sound, so that's good. I'll have a word to our neighbours later to find out about her vet's history, then see about getting a microchip stuck in her back (if she doesn't already have one). This is something I heard about relatively recently (none of our cats had them when I was a kid) - the idea is that the chip stores an ID number, and this can be looked up in a central database to get our address. So, if the cat wanders onto a removals van or something, and winds up at the opposite end of the country, the local animal shelter can do a "cat scan" then get in touch with us.