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<-   To check an equation   ->

The first trick to checking an equation is to use units :
To derive the time necessary for light to come from Neptune (4*10^9 km), at a speed of 300000 km/second, is it :
time = distance/speed or time = distance x speed ??
All you need is to replace this with units :
second=kilometres/(kilometres/second) or second=kilometres x kilometres/second
to see if the first equation is the correct one.
time=4*10^9/300000=13333 seconds, we divide that by 3600 (seconds/hour) to get 3,7 hours.