In problem 9, it is not clear if the people from the previous day show
up the next day as well, or if only the people they invited show up. I
thought the meaning was that only the invited people show up. In that
case the recurrence is very simple ($a_n = 2 a_{n-1}$), which makes me
suspect that this wasn't the intended meaning.

In 12c, is it assumed that $n\leq 100$?

In 13d, there is a sudden implicit jump from $10$ vertices to $n$
vertices. Is 13c also about $n$-vertex graphs, or still 10-vertex?

In general, the test seemed very simple. The only part that took me
some time was 2b, simply because I am not used to doing Euclid's
algorithm by hand (especially without a calculator). Also, problem 7
seemed slightly involved (though still very straightforward).