It... is just multiplication. And can't talk about GP's experience, but I can tell you that going through scientific schooling and engineering schools in the French system you'll learn exactly how to calculate the math and never have a single example such as mentioned above.

We're here to build bridges, not count stashes of money after all!

You'd probably get those if you went through "economic studies" (which is a different track and where math includes a lot more statistics even in high school).