The definition of the speed of light is:

299,792,458 meters per second

and

one second equals 109 nanoseconds.

Set it up like this:

(299,792,458 m s¯1) x (1 s/109 ns)

0.299792458 meters per nanosecond

One meter is defined as 39.37 inches.

(0.299792458 m ns¯1) x (39.37 in m¯1) = 11.8 inches per nanosecond

One foot is 12 inches. Light travels approximately one foot in one nanosecond.