I bet even Stephen Hawking would want to use a calculator on that test!
I think that it's not just testing mental arithmetic, cause if it was they may as well just give us lots of big numbers to multiply and divide in our heads, instead of using all the statistical tables and graphs and things. The tables are there to see whether we can understand "complex" numerical data (ok, it's not cosmology, but you do have to think) quickly and figure out what the right thing to calculate is in the first place. There's also a bit of a test of data checking skills in there, for instance, you have to make sure you're looking at the figures for 1997 not 1998, etc!
I doubt the actual calculation itself is really the bit they're trying to test, in this particular case. If they wanted to test whether we could do mental arithmetic then an online test would be a daft way to do it, because as Someone_Else said, there's no way of policing it and some people would just use a calculator anyway! Maybe at some other point they'll check our mental arithmetic - they seem to have changed the application process recently anyway, judging by what people have been saying.
I don't mean to rant at Something_Else's comment, just that I reckon that the kind of things they're testing might've changed.
To be honest, I think the number of applicants must be going up, and this is probably a relatively cheap way for them to weed people out right at the start, so they don't have to give out so many free lunches at the selection test day!
