Answer
2.5 s.
Work Step by Step
The Earth-moon distance is $3.8 \times 10^{8} m$, and the round-trip distance is twice that.
If c denotes the speed of light, then $d = 7.6 \times 10^{8} m = ct$. Solve for t = d/c.
$$\frac{7.6 \times 10^{8} m }{300,000,000 m/s} = 2.5 s$$
