C1744 Calculated interest

"Hehe, wait, wait. This game of numbers is really hard to deal with, I feel a little dizzy. Twenty billion is still 12 billion if you give him eight billion. Right, right. In the future, I'll only be able to charge you the interest of the twelve billion, and I'll give you the interest of the eight billion to escape. Hehe.

Alright, if we calculate it this way