The best example I recall of his innate math skills was one time
when he got a sunburn on his forehead in the summer.
He started to
wonder what fraction of the sun's total energy it had taken to fry
the skin on his forehead.
So, from scratch (pre-WWW), he reasoned that if you took a sphere
with the radius of the earth's orbit, and calculated the area of
that,
the proportion of that to the area of your forehead would be
the answer. Which is correct.
So he looked up the formula for the area of a sphere, and the
radius of the earth's orbit, and got the fun answer.
93 million miles, 4*pi*r^2, and a 12 square inch forehead gives
you 12 sq inches / 1.09e17 sq miles.
Since there are 4.014e9 sq inch in a sq mile, the result is
2.75e-26. Which is entertainingly small.
I do think his fancy calculator could do scientific notation.