For a reversible reaction $\mathbf{R}\rightleftharpoons \mathbf{P}$, at constant temperature, both the forward and the backward reactions are first order elementary reaction with rate constant $k_f$ and $k_b$, respectively. At time zero, the concentration of $\mathbf{R}$ is $[\mathbf{R}{]}_0$ and the concentration of $\mathbf{P}$ is zero. At any given time $[\mathbf{R}]$ and $[\mathbf{P}]$ are the concentration of $\mathbf{R}$ and $\mathbf{P}$, respectively. If $k_b=4k_f$, the correct graphical representation of the reaction is