If a strand of DNA has 17% T, what percent will
be G? C? A?

If a DNA strand has 17% Thymine (T) the percentage of the other nitrogenous bases will be 33% of Guanine (G), 33% of Cytosine (C) and 17% of Adenine (A), according to Chargaff's law.

Explanation:

The percentage of nitrogenous bass in a DNA strand can be established by knowing the percentage of one of the bases present in the molecule, in this case knowing that Thymine corresponds to 17% of bases.

DNA has the nitrogenous bases adenine (A), guanine (G), thymine (T) and cytosine (C).  The bases of one chain are matched with the bases of another, according to the complementarity of the nitrogenous bases, where:

According to this, DNA molecule there is as much T as A, and an equal amount of G and C, and the percentages of nitrogenous bases can be calculated according to Chargaff's rule.

Chargaff deduced that in a DNA molecule the ratio of purine:pyrimidine is 1:1, so there must be the same amount of thymine as adenine and a similar amount of guanine for the cytosine, according to the complementarity of the bases.

Calculating the percentages, according to the law of the base pair, if in a DNA chain there is 17% of T, in the molecule there is:

"do a testcross with a homozygous recessive plant."

nucleus

would be most reasonable answer tbh