# Question #e9b89

##### 1 Answer

#### Explanation:

The idea here is that you can use the **speed of light** as a **conversion factor** to help you calculate the time needed for light to travel a distance equal to the circumference of the Earth.

So, the speed of light is given to you in *meters per second*, *miles* to *meters*.

#24900 color(red)(cancel(color(black)("mi"))) * overbrace((1.609344color(red)(cancel(color(black)("km"))))/(1color(red)(cancel(color(black)("mi")))))^(color(blue)("miles to kilometers")) * overbrace((10^3"m")/(1color(red)(cancel(color(black)("km")))))^(color(darkgreen)("kilometers to meters")) = 4.0073 * 10^7"m"#

Now set up the speed of light as a conversion factor to find

#4.0073 * 10^7 color(red)(cancel(color(black)("m"))) * "1 s"/(2.998 * 10^8color(red)(cancel(color(black)("m")))) = color(green)(bar(ul(|color(white)(a/a)color(black)("0.134 s")color(white)(a/a)|)))#

The answer must be rounded to three **sig figs**, the number of sig figs you have for the circumference of the Earth.