A radar sends an electromagnetic signal of electric field $\left(\mathrm{E}_0\right)=2.25 \mathrm{~V} / \mathrm{m}$ and magnetic field $\left(\mathrm{B}_0\right)=1.5 \times 10^{-8} \mathrm{~T}$ which strikes a target on line of sight at a distance of 3 km in a medium. After that, a pail of signal (echo) reflects back towards the radar vit1i same velocity and by same path. If the signal was transmitted at time to from radar. then after how much time echo will reach to the radar?