Units for verifying an equation

A smart way to verify an equation is to use units:
How do you find out the time light takes to reach us from Neptune which is 4 billion kilometres away (4 x 10⁹ km), at the speed of 300,000 (3 x 10⁵) kilometres per second?
Is it:

$$ \begin{align} \text{temps} &= \frac{\text{distance}}{\text{vitesse}} \\ \text{or} \\ \text{temps} &= \text{distance} \times \text{vitesse} \space \text{?} \end{align} $$

??
Simply replace by units:

$$ \begin{align} \text{second} &= \frac{\text{kilometre}}{\frac{\text{kilometre}}{\text{second}}} \\ \text{or} \\ \text{second} &= \text{kilometre} \times \frac{\text{kilometre}}{\text{second}} \\ \end{align} \\ $$

to see that the first solution is the right one.

that you divide by 3,600 (seconds per hour) to obtain 3.7 hours.