If the base adenine constitutes $\displaystyle 31$% of an isolated DNA fragment, then write what will be the expected percentage of the base cytosine in it. Explain how did you arrive at the answer given.

Amount of A = T and G = C and A+G == C+T / ratio between Adenine and Thymine and ratio between Guanine and Cytosine are constant and equals one / A / T = G / C = $\displaystyle 1$, If A = $\displaystyle 31$ % then T = $\displaystyle 31$% ∴ A+T=$\displaystyle 62$% C + G= $\displaystyle 100$-$\displaystyle 62$=$\displaystyle 38$% ∴C = $\displaystyle 38$ / $\displaystyle 2$ = $\displaystyle 19$ %