In a strand of DNA, if the percentage of thymine is 30%, what would the percentage of cytosine in the same DNA strand be?

Well in DNA the complimentary pairs are Adenine and Thymine, Cytosine and Guanine - Whatever Thymine has Adenine must have the exact same (So in this case Thymine has 30% this must mean Adenine also has 30%) so if there is 40% remaining this must be split between the pair Cytosine and Guanine which would give 20% each

Rna polymeras should attache to the promoter.