If there are 2000 nucleotides in a strand of dna and 600 contain the nitrogen base cytosine, how many nucleotides contain adenine?

The correct answer is 400.

There is a complementary bounding in each DNA helix between nucleotide bases.  

There are two types of nitrogenous bases :the single-ring (thymine and cytosine) also called pyrimidines, and the double-ring bases (adenine and guanine), also called purines. Two DNA strands are complementary paired: cytosines form three hydrogen bonds with guanines, and adenines form two hydrogen bonds with thymines (C and G is one base pair and A – T is another).

So, if there is 2000 nucleotides in double stranded DNA, C=600 and G=600. That means that A+T=800, A=400 and T=400