A stretch of double-stranded DNA has 50 base pairs, and 60% of the base pairs are G-C pairs…

In double-stranded DNA, A pairs with T and G pairs with C. So $A = T$, $G = C$ and $A + T + G + C = 100\%$. An A-T pair has 2 hydrogen bonds and a G-C pair has 3. A strand of $n$ nucleotides has $n - 1$ phosphodiester linkages. A chain of nucleotides has a molar mass equal to the sum of the nucleotide residues plus one water.