2 millisec = 2/1000 sec = 2x10*-3 seconds = Time
Speed = Speed of sound = Speed of Radio wave = 300000000 m/s =3x10*8 m/s
Distance = Speed x Time
= (2x10*-3) x (3x10*8) = 6,00,000 m = 600 km
1nm = 1.85 km
Hence, 600km = 600/1.85 = 323.9 nm
This distance is to and fro distance
Hence, 323.9/2 = 161.95 nm