If a cells nucleus has 55% adenine bases, how many cytosine bases will be present?

If a cell's nucleus has 55% adenine bases it also has 55% thymine bases, so the percentage of cytosine bases is 45% as same as guanine bases.

This is known as Chargaff's rule which states that DNA should have a 1:1 ratio of pyrimidine and purine bases (the amount of guanine should be equal to cytosine and the same thing with adenine and thymine).

DNA molecule globally has percentage base pair equality: %A = %T and %G = %C.