Bonus Project Euler problem

This section is about official bonus problems of Project Euler.

These sometimes pop up after you solve another problem, and it is not well documented when they show up.

According to their copyright statement, these problems are copyrighted unlike the others, so we can't reproduce them here. But we can give answers I believe.

A partial list can be found at: github.com/igorvanloo/Project-Euler-Explained/tree/main/Bonus%20Problems