Cytosine makes up 38% of the nucleotides in a sample of dna from an organism. approximately, what percentage of the nucleotides in this sample will be thymine?

Cytosine will always base pair with Guanine (assuming no mutations present).
If Cytosine makes up 38% of DNA, Guanine will also make up 38% of DNA.

Adenine will always base pair with Thymine (similarly assuming no mutations present).
The remaining percentage that is not Cytosine nor Guanine will be 100% - 38% - 38% = 24%
Adenine and Thymine together make up 24% of DNA.
Adenine and Thymine should have the same percentage.
Adenine % = Thymine % = 24% ÷ 2 = 12%

Hope this helps! :)