The definition of the speed of light is:

299,792,458 meters per second

and one second equals 10^{9} nanoseconds.

Set it up like this:

(299,792,458 m s¯^{1}) x (1 s/10^{9} 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.