Airplanes $A$ and $B$ are flying with constant velocity in the same vertical plane at angles $30^{\circ}$ and $60^{\circ}$ with respect to the horizontal respectively as shown in figure. The speed of A is $100 \sqrt{3} \mathrm{~ms}^{-1}$. At time $\mathrm{t}=0 \mathrm{~s}$, an observer in A finds B at a distance of 500 m . This observer sees B moving with a constant velocity perpendicular to the line of motion of $A$. If at $t=t_0, A$ just escapes being hit by $B, t_0$ in seconds is: