Question

How many nanoseconds does it take light to travel 2.50 feet in a vacuum

Answer 1

A handy-dandy "rule of thumb" that I and many other engineers
have been using for years is:       

                                     1 nanosecond ===> 1 foot. 

Let's figure it out and see whether that's even close:

Speed of light in vacuum = 299,792,458 meters per second.

         (299,792,458 m/s) x ( 1 sec / 10⁹ nanosec) x (3.28084 ft/meter)

     =  (299,792,458 x 1 x 3.28084 x 10⁻⁹)  ft/nanosec

     =            0.983571  ft/nanosec  .

       That's  1.64%  less than  1.000000 .
       The engineer's handy-dandy rule of thumb is valid.

2.5 feet would take

           (2.5  x  0.983571 sec)  =   2.45893 nanoseconds