To form a DNA strand nucleotides are joined together - sugar of one to the phosphate of the next - an the nitrogenous bases stick out from this sugar-phosphate backbone. Hydrogen bonds between the bases on opposite DNA strands hold the double-stranded DNA strict together. The hydrogen bonds formed between two strands of DNA only 'fit' together a certain way, given us standard base pairing rules.
You have a double stranded DNA molecule with 100 nucleotides (both strands combined have 100 nucleotides). You know 30 of those nucleotides are T nucleotides. How many C nucleotides are there?