In Fig. $37-9$, observer $S$ detects two flashes of light. A big flash occurs at $x_{1}=1200 \mathrm{~m}$ and, slightly later, a small flash occurs at $x_{2}=480 \mathrm{~m}$. The time interval between the flashes is $\Delta t=t_{2}-t_{1}$. What is the smallest value of $\Delta t$ for which observer $S^{\prime}$ will determine that the two flashes occur at the same $x^{\prime}$ coordinate?