$\begin{align} & \frac{1}{3}=\frac{4}{x-1}\to x-1=12\to x=13 \\ & \frac{1}{3}=\frac{5}{x-1}\to x-1=15\to x=16 \\ & \frac{1}{3}=\frac{6}{x-1}\to x-1=18\to x=19 \\ \end{align}$