Fibonacci was one of the most important mathematicians of the Middle Ages. He was the person who first realized the implications of the Arabic numeral system and promoted it with his publication of Liber Abaci, which described it. If you want to understand the implications, try figuring out what you get when you divide CXXIV by XXXI, entirely in Roman numerals, then understand that was the only system medieval merchants had available until Fibonacci's book appeared.
Fibonacci also did a fair amount of original work in mathematics, publishing a number of books. There is a link to an article on him below.
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A person was considered old when they were in their 40s.
Yes, it was common and often considered a compliment
Merchants, doctors, professors, lawyers, and some prosperous tradesmen were considered to be middle class in the British Colonial period in America
No. 1994 is considered mid-'90s with 1995 and 1996. The exact middle of the 1990s is January 1, 1995. That implies 1993 is as close to the middle as 1996. (This is why 1993 is also sometimes considered mid-'90s.) And it implies 1994 is as close as 1995 to the middle. If you're dividing the '90s between early and late, and not considering a middle, then you'd place 1994 in the early section. (The early '90s would thus be 1990 to 1994, and the late '90s would be 1995 to 1999.) But this is rarely done.
Since the middle ages had no knowledge of the brain or its functions there were several ideas about intelligence and the soul. The heart was considered to be one of the places where these were located.