30 percent (The 20 percent of thymine would pair with 20 percent of adenine for a total of 40 percent A and T. 100-40 = 60 percent remaining guanine and cytosine nucleotides. 60 / 2 = 30 percent guanine and 30 percent cytosine.) A plant's DNA has nucleotides that are 20% thymine.
hopefully that helps