Equations with variables on both sides

I am looking for the equation for this problem. I know the answer is 105 minutes. I see the answer as a ratio, but I need to set up an equation for the solution.

Selma's class is making care packages to give to victims of a natural disaster. Selma packs one box in 5 minutes and has already boxed 12 boxes. Her friend Trudy packs one box in 7 minutes and has already boxed 18 boxes. How many more minutes does each need to work in order to have packed the same number of boxes?

 

The key to this is to determine the number of boxes they will both have made. Call this N, so that Selma must make N - 12 more and Trudy N - 18.
This will take 5(N -12) minutes for Selma and 7(N-18) for Trudy.
You find N from the fact that these are equal.
Hence N = 33, and so Selma needs 21 more and Trudy 15.
As you say, this takes each of them 105 minutes

 

We set them equal to each other and solve for N.

 

What information 8n the statement hinted that
N - 12 and N - 18 is needed as part of the equation?