If there are 30 bases of cytosine in DNA, then how many bases of thymine are there?

Ok, so complementary base pairing occurs within DNA -- cytosine and guanine, adenine and thymine. So if there is 30 bases of cytosine then there MUST be 30 bases of guanine which then equals 60. So to make a whole number of 100, we have 40 left, again due to the complementary base pairing, there must be the same number of thymine bases as adenine bases. So, 40/2 = 20, therefore, there are 20 bases of thymine in this example of DNA.