The reason why people can't do that calculation in their heads is because of our reliance on computers and calculators.

http://www.youtube.com/watch?v=xguMYWW8EAU

In fact you can pull up articles with a quick search about how people can attain pretty good mental math capabilities through training. It's hard work, but it's proof of the adaptability of the human mind.

I don't know I already thought the computer was the first "lever" for the mind that we've had. The computer has been pretty game changing for society, has it not?