If 15% of the nitrogenous bases in a sample of dna from a particular organism is thymine, what percentage should be cytosine

So Adenine and Thymine are base pairs so they have to be equal to each other. So Adenine and Thymine are both 15% or 30% together.

This means there is still 70% left.

The next base pair is Cytosine and Guanine. they also have to equal to each other. Since there is 70% left you divide that number by two and you get 35%.

This means 35% is Cytosine in the strand of DNA