Lesson Learned

Oh, you're sorry are you?

A/N: Am I maybe a little too obsessed with this story right now? Probably, but when inspiration hits like a truck, you just have to go with it.

***

Mitsuko felt every muscle tighten uncomfortably as she watched and listened to her dad explaining how he was going to kill Koro-sensei.

She and the rest of the class were watching from the window while Koro-sensei stood at the centre of the classroom surrounded by five desks with five different workbooks while her dad paced around him.

"I've prepared five textbooks," the Principal explained. "And five hand grenades. Four of them are anti-sensei grenades." He gestured to the grenades. "While the fifth...is a normal grenade."

Mitsuko's stomach lurched. "What?!"

Her dad remained calm and picked up a grenade. "They are completely identical in appearance and odour." His finger curled around the pin. "And I've made it so that they explode the moment you pull the pin and release the lever."

Karasuma and Bitch-sensei watched from around the doorway apprehensively, staring at her dad like he was insane.

"Why the hell are you using a real grenade!?" Mitsuko yelled, leaning against the open window. "Dad, this is—"

"All assassination requires some level of risk, Mitsuko," her dad replied lightly like it wasn't his life on the line. "And if you were paying attention, you'd realise it's a well-considered one."

"That's doesn't..." She grimaced, running the numbers in her head. From a logic standpoint, the math pointed in her dad's favour since it was  20% chance...

20%...

Rio reached out and held her hand. "Don't worry, he's not actually going to get himself killed," she whispered.

Mitsuko's stomach twisted. "Right..."

"And now I'll pull the pins." He pulled out the pins and put then under the textbooks. "Set then in the proper workbooks pages. And pinch them so that the workbooks prevent the levers from moving." He explained. "Now open the workbook and solve the problem on the upper right." Koro-sensei was sweating. "However, he is not allowed to move until he solves the problem."

Hara looked alarmed. "But-but the moment he opens the workbook the lever will—!"

"Yes!" Dad smiled brightly. "He'll almost certainly bear the brunt of the explosion!" he said enthusiastically. "First, he'll solve four books, and then I'll do the remaining one." He leaned close to Koro-sensei menacingly. "And here's the gamble. If you can kill me or make me give up, I'll allow you and Class E to Stay here." He smiled wider. "Now let's see...Terasaka. I want you to calculate the probability that Koro-sensei will win."

Mitsuko looked over at Terasaka who scowled.

"...Well...if an anti-sensei grenade blows up, it won't do anything to you so to kill you, he has to make sure he doesn't get the real grenade." He grimaced, taking a moment to think. "4/5 X ¾ X 2/3 X ½ + 1/5. Only 20%."

"Correct!" The Principal replied brightly.

Mitsuko's eyes widened in surprise. "You did listen!" She smiled brightly. "You actually looked at my flashcards on percentages!?"

Terasaka looked annoyed. "Only because you were shoving them in my face!" He glared at her father.  "Also! The octopus has to survive four dead grenades in a row. But if things get too hairy for you, you just have to give up and walk away without a scratch!" He gritted his teeth. "This game is totally rigged in your favour!"