Inosuke and Tanziro are overjoyed because Mr.Haganezuka is bringing their sword of revenge. Because they broke their old sword while trying to kill Spider Demons. After receiving the new sword, Inosuke begins to break it again to give shape like his old one, which angers Mr.Haganezuka because he cannot bear that his creation is being destroyed. He raged at Inosuke. After Tanjiro tried for a long time finally, he calmed down. But for this crime, however, he punished Inosuke with a mathematical problem because he knew that Inosuke was very weak in mathematics.
This is a companion discussion topic for the original entry at https://toph.co/p/inosuke-and-trailing-zero