I kind of have a 'mode' I get into for these things, so if I came into an interview I would probably be expecting "tell me about a time when you..." questions. If you threw a basic question like "what's Kirchoff's current law?" I'd probably be able to answer it, but if you threw maths at me I'd be a bit caught off guard and panic then spend the rest of the interview thinking "dammit, I am screwed". Also, working with decimal places in my head can be a bit of a pain sometimes, so I usually use a calculator even for basic questions like the one you asked.
I guess my point is that it's hard to get an accurate idea of what the person would actually be like on the job from 1 interview, especially if they have been through wankey HR and group interviews. You may catch people off guard and make them look stupid and feel self conscious about things they wouldn't normally have trouble with.
Thanks for this topic though, it is interesting to get the perspective of the guy across the table.