Airplanes $A$ and $B$ are in the same vertical plane and flying with constant velocity. At time $t=0\text{sec}$, an observer in $A$ finds $B$ at a distance of $500\text{m}$ and moving perpendicular to the line of motion of $A$. If at $t=t_0\text{sec}$, $A$ just escapes being hit by $B$ , $t_0$ in seconds is