At a given point in its orbit, the moon is 240,000 miles from the earth. How long does it take for light from a source on earth to reach a reflector on the moon and then return to earth? The speed of light is 3.00 x 108 m/sec.

I know:


1609 meters= 1 mile
I need to set up an equation with some kind of ratios.

Well, you know the distance and speed of light. How long does it take to get to the moon? Double your answer or use 480,000.

You need to compare apples to apples. Convert your 'miles' figure and your velocity into Km first.

Miles per second is the ratio and converting it to km per second is also a ratio. Doing it both ways gives you a comparison and the ratio of km to miles.