The first trick to checking an equation is to use units :
To derive the time necessary for light to come from Neptune ( km), at a speed of 300000 km/second, is it :
or x ??
All you need is to replace this with units :
or x
to see if the first equation is the correct one.
seconds, we divide that by 3600 (seconds/hour) to get 3,7 hours.
