I don't know about the Prison Escape one, but I'll offer some general advice on planex's. At my main board not a single person in the group managed to get the best answer; and I know that we didn't all fail. So clearly you're not automatically expected to get a perfect answer (although obviously it won't hurt). With the one I did, there was a dead cert answer, maybe a couple of solutions which could potentially work and lots which definitely wouldn't. I'm not sure about "fiddly" calculations - you're not doing calculus, just distance/speed/time and maybe some addition, so if you're struggling maybe some mental arithmatic practice is in order.
My always-dubious advice, in no particular order of importance. Some of it's from my own experience, some of it regurgitated from other people:
- Get your calculations spot on.
- Manage your time well, make sure you show on your answer sheet that you've at least looked at a couple of alternative solutions.
- Never assume you have anything which isn't on the briefing.
- There could be some gambles thrown your way: There may be a road, but will anybody be driving on it? There may be an airfield, but will the pilots be present/willing/sober enough to fly you where you want to go? If you're going to gamble, be prepared to justfy it in the debrief.
- Don't sweat it if you don't have the dead cert solution, so long as you have something that works, and that you can defend against some scrutiny.
- Write down everything you do. If you didn't write it down, it didn't happen. Keep your notes concise but clear.
- Finally, don't start your debrief with "I know that I've totally messed up this plan, but...". The DS will be glad to pick holes in your plan without any assistance from you.
Gone off on a tangent there, but I hope this is useful to someone.