The radio waves move at the speed of light: [tex]c=3 \cdot 10^8 m/s[/tex], and they cover a distance of [tex]S=120 km= 1.2 \cdot 10^5 m[/tex] to reach the people, so the time they take to cover this distance is
[tex]t= \frac{S}{c}= \frac{1.2 \cdot 10^5 m}{3 \cdot 10^8 s}=4 \cdot 10^{-4} s = 0.4 ms [/tex]
Instead, the sound waves move at speed of sound: v=350 m/s and they cover a distance of S=3.6 m, so the time they take to reach the people is
[tex]t= \frac{S}{v}= \frac{3.6 m}{350 m/s}=0.0103 s=10.3 ms [/tex]
Therefore, the difference in time between the two messages is
[tex]\Delta t = 10.3 ms - 0.4 ms=9.9 ms[/tex]
