It's the final episode. What does everyone think about math now?
Nina is confused why her sister, Machida Kina, is here. Kina explains that she's here to fight Nina. Kina explains to Kazuki and Sayuri that Nina came to this school to find Kina and find out what happened to her after she became Math Boss. Nina is still confused and Kina says that she wants to rule over all the mathematicians in the world. (What? Why?) Kina wants to wipe out all the math schools using her position as Math Boss, even this one. Kina didn't want any mathematician to get in her way of ruling the world.
Nina is in shock. The sister she knew would never act like this. She would be so happy and excited about math, but the person she's looking at has dead eyes. Kina says it looks like she has to fight Nina and destroy her. Nina accepts the challenge and will defeat this impostor. Kina is so confident that she'll win that she tells Nina that this will be her last battle.
Team Nina and Kina are in a classroom, where Kina explains that the book she's holding holds 1200 years of math history, including 20,000 math problems that have been unsolved during all those years. Kina says that she'll choose a random unsolved math problem and Nina accepts. The problem Kina chooses is "Clearly prove that 1 + 1 = 2". Kazuki comments saying that everyone knows that answer is correct, but how would you go about proving it.