Complex sentence: The nines' complement of a decimal digit is the number that must be added to it to produce 9; the complement of 3 is 6, the complement of 7 is 2, and so on.
Split into simpler sentences: The nines' complement of a decimal digit is the number that must be added to it to produce 9. The complement of 3 is 6, the complement of 7 is 2, and so on.