How many years is three score and nine centuries?

Three score and 9 centuries would be 3 X 20 + 9 = 69 centuries, or 6,900 years. A score is simply 20, just as a dozen is 12. A score of years would be 20 years, four score and seven Apples would be 4 X 20 + 7 = 87 Apples.