The motion of the light is a uniform motion with constant speed [tex]v=3 \cdot 10^5 km/s[/tex], therefore we can use the basic relationship between speed, space and time:
[tex]v= \frac{S}{t} [/tex] (1)
where S is the distance covered and t is the time taken. The light takes t=1.3 s to travel from the moon to Earth, therefore by rearranging eq.(1) we can find the distance between the Moon and the Earth:
[tex]S=vt=(3 \cdot 10^5 km/s)(1.3 s)=390000 km=3.9 \cdot 10^5 km[/tex]
