Adenine pairs with thymine and guanine pairs with cytosine.
100 nucleotide pairs are a total of 200 nucleotides. Because of base pairing, if there are 25 adenine there must also be 25 thymine. This leaves 200–50 = 150 nucleotides to be divided evenly between guanine and cytosine.
Nitrogenous base pairs are joined by hydrogen bonds.