america's favorite f-words: "founding fathers"

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Amendment VII

In suits at common law, where the value in controversy shall exceed twenty dollars, the right of trial by jury shall be preserved, and no fact tried by a jury, shall be otherwise reexamined in any court of the United States, than according to the rules of the common law.

The question i have is, what does twenty 1789 dollars come out to be in today's currency?

Oh, and another question: why twenty dollars?