[AmyJ]

This seems rather elementary but it's driving me crazy.
In lecture my professor wrote down the following:

There are 6 students for every professor in the university.
This means that 1P is equivalent to 6S.

He then said the equation to represent this equivalency is:
S = 6P

Of course the answer makes sense.  Plug in 3 for P and you get 18 students = 3 professors which is correct.
But how do you get the equation from the equivalency?