Chemistry - Calculating the molarity of DNA in a cell

There is one particle of dsDNA in the cell. Divide by $N_\mathrm{A}$ to get the amount of substance, and divide by the volume to get the concentration matching the concentration given in the answer.

\begin{align} V_{\mathrm{cell}} &= 4 \cdot {(\pu{0.5 μm})}^{3}\\[0.5ex] &= \pu{0.5 μm3}\\[0.5ex] &= \pu{5E-16 L}\\[3.5ex] N_{\mathrm{A}} &= \pu{6E23 1//mol}\\[3.5ex] n_{\mathrm{DNA}} &= \frac{1}{N_{\mathrm{A}}}\\[0.5ex] &= \pu{1.7E-24 mol}\\[3.5ex] c_{\mathrm{DNA}} &= \frac{n_{\mathrm{DNA}}}{V_{\mathrm{cell}}}\\[1.5ex] &= \frac{\pu{1.7E-24 mol}}{\pu{5E-16 L}}\\[1.5ex] &= \pu{3E-9 mol//L} \end{align}