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If you are thinking of the word 'score' as a span of life, it is of biblical origin, Leviticus 12 and Psalms 90 which quotes 'there is a use of it that refers to a span of our lives' - 'the days of our years are three score years and ten'. Also borrowed by Shakespeare in Macbeth as 'Three score and ten I can remember well,' The span of life was then considered to be three score and ten, in other words 70 years
Eight score and ten is 170-years. A score equals twenty-years. The most famous use of the word score, is in the Gettysburg Address. Score is a slang term for a twenty-dollar bill.
A decahedron is a solid shape with ten flat faces.
There is no special word. You could use "quarter century".
It's probably in base ten because we use base ten for all our OTHER work in numbers as well. We learn to write numbers in base ten when in elementary school, and use base ten throughout our lives even if we aren't scientists or mathematicians or students or professionals who need to use scientific notation. So keeping scientific notation in base ten makes it easier for everyone to learn and read.