The ratio 19 to 83 or 19:83 is also 19/83. It can not be simplified, 19 & 83 are both prime.

To answer your question, "What part of speech is "the"?"

"The" is a definite article which is an adjective describing an unspecific noun, like the dog or the table.

You can find out more here:

http://www.grammar-monster.com/glossary/definite_article.htm