@ehBeth,
In my experience, except in rare circumstances, human beings can learn to do the level of calculation they need to.
I have always been focused on reason which meant I didn't do so well in math. Then the school had the good sense to put me into an independent study class. Once I was given problems that were interesting to solve I excelled since I would do the work I wanted to do. I did calculus in high school this way.
As a teacher I worked with many very bright kids who were the same way. I am now a computer engineer and many of us in that industry had similar experiences. Reasoning above calculation certainly works well in my highly technical field. Of course none of us are at the extreme of not being able to calculate correct change.
A few years ago I found a hobby that requires the ability to do quick calculation when I started playing poker. To play poker well I need to be able to consider possibilities and come up with a fairly accurate estimate of the probability of each one. These estimates need to be done in my head in a matter of seconds so I can make a quick decision at the table.
Poker is the first thing I have ever done where I actually need to focus on calculation. I have even taken to memorizing tables. Before this I never memorized anything including multiplication tables.
Funny I found that when I had a good reason to be good at calculation, I figured it out pretty quick and did whatever I had to do to learn something I felt was worthwhile.
I think this is a sign of intelligence. Many really bright kids have trouble being motivated at rote exercises that don't have an immediate purpose. The key with this type of kids is to focus on problems that interesting.
Inspiring curiosity is the most important part of education process.