In DNA, Guanine pairs with Cytosine, and Thymine pairs with Adenine. This tells us that there are equal numbers for each Guanine and Cytosine, and Thymine and Adenine. When there's 35 Guanine, there should be 35 Cytosine that goes with it. 120 - 2(35) = 50. The remaining 50 nucleotides are Thymine and Adenine, and since they are the same number, divide 50 by 2. 50 ÷ 2 = 25. There are 25 thymine in the sample of DNA.
