Your First Equation

Every quantity in a problem gets a letter: the distance is d, the time is t and the speed is v. The rule that connects them, in letters:

$v = \frac{d}{t}$

It says: the speed is the distance divided by the time.

The board asks for the rule in letters first, never the numbers: v = d / t, not v = 150 / 10. The same rule then works for every problem, and you can see which quantity went where.

Then, in Solve, the numbers go in and you work it out: v = 150 / 10 = 15. The last line needs its unit, because 15 alone does not say 15 what: v = $15\ \mathrm{m/s}$.