@christianp@ColinTheMathmo Eh, I thought they were kinda fun. In every case it was an exercise in "can you spot the trick?" and enough of the tricks were at least semi-interesting to hold my interest.
(Except maybe for 16, which feels like it should be straightforward but somehow isn't working out for me so far. I suspect I've done something stupid or missed something easy, and it does fall under the rubric of "spot the trick that makes it simple" even though I either haven't spotted it or have screwed it up.)
I'm old and slow (at least compared with when I was younger and quicker) so I certainly didn't do them all in 20 minutes.
ColinTheMathmo