I have to agree. This probably isn't all that difficult to work out when you're in the zone, home alone but in an interview is completely different. You're in a completely different state and it's hardly reflective of a normal working environment.

I think if you are going to give this to people in an interview you should a) make sure you've told them to expect puzzles and b) give them a few questions, some as a warm up with plenty of time to settle in.