The radio waves travel at speed of light:
[tex]c=3 \cdot 10^8 m/s[/tex]
The distance of Mars from Earth when it is at the farthest point is:
[tex]S=401 mil. km = 4.01 \cdot 10^{11} km[/tex]
And since the motion of the radio waves is a uniform motion, we can use the basic relationship between velocity, space and time to find the time the waves take to reach Mars:
[tex]t= \frac{S}{v}= \frac{4.01 \cdot 10^{11} m}{3 \cdot 10^8 m/s}=1337 s [/tex]
