Interesting mathematics?
This is something else. My son gave me here.
He goes into the room and says

C: I calculated here that if we buy a new toilet, it will repay itself in 75 years.
I: Is it like this?
A: very simple: if you buy a toilet of a special design, preventing the formation of splashes we will save money on the paper, which we throw into the toilet every time to prevent the unpleasant "splash".

I am in the car. The Milky Way =)