Yes and no. Yes we use a 100%, no we generally don't use those 100% on one task. Eating, breathing, seeing, remembering,… all the unconscious stuff of just "being" there generally take up a large part (even up to 80%) of your brain capacity.
Concentrating or focussing on one task CAN really make us use a bit more (and that's why we don't observe our environment as well as we otherwise would, forget things or even change our breathing pattern when doing that) but never 100%. Sometimes when we panic and everything seems to slow down is because you just kicked your brain in overdrive and put everything towards "surviving" whatever made you panic, which is as close to the 100% usage as we can get.
I disagree. If we used all brain capacity, in terms of processing, then we wouldn't have people becoming worse in Mathematics, over the years.
After all. Mathematics are but the logic of the Human mind. Every sciences out there, take use of Mathematics. Even when we decide to take our clothes on in the morning, it is a process of Mathematical calculation being done.
However. Most people become "slower" at Mathematics, after just a few weeks of "not" doing it. This is an indicator that the Brain is NOT doing its full potential. Hence my opinion.