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.