Navin has answered my question. Unfortunately, there is no way to comment on his blog (It says “comments”, but clicking there does not get me anywhere.) So I have to answer here.

Firstly, he still hasn’t explained what he meant by throwing that equation at me. What did that equation “show clearly”?

Then he answers my question saying “Productivity”. That of course, explains everything. So I need an extra half a pane of glass next year. So the simple answer is to produce more. You have to do so with the same number of workers, so the only way to do it is… increase productivity! Yes, it is all clear now. Mathematics is the solution to all the world’s problems.