If the period of a Repeating Decimal for has an Even number of digits, the sum of the two halves is a string of 9s, where is Prime and is a Reduced Fraction.
See also Decimal Expansion, Repeating Decimal
Rademacher, H. and Toeplitz, O. The Enjoyment of Mathematics: Selections from Mathematics for the Amateur.
Princeton, NJ: Princeton University Press, pp. 158-160, 1957.