This one was just … kinda fun.

Sometimes you gotta let the nerds be nerds.


.


One of the major things that Noa Kaiba and Ryo Bakura shared with each other was a love and appreciation for numbers. They spent more than a fair amount of time, whenever they met for food or drinks or anything in between, discussing them. The context was often random, but it always came down to numbers. They talked strategy games; they talked gambling; they talked statistics. Ryo would approach Noa, trying to work out the specifics of some tabletop game that he was struggling with. Noa would approach Ryo, trying to puzzle out some facet of human nature.

Every time, eventually, it came down to the numbers.

Sometimes, this would lead to one or the other explaining something that his companion just couldn't wrap his mind around.

"That cannot be right." Ryo was shaking his head, fumbling with his phone's calculator. "There are 365 possible birthdays, 366 if we count a leap year. A classroom of thirty people . . ."

"There is a 70 percent chance you'll run into the same one twice," Noa said confidently. "It's called the Birthday Problem for a reason. People struggle with it. But if you break it down to the raw numbers, it makes things a lot clearer. Well, to me. And to Aniki. Mokuba still thinks we're both pranking him."

Noa pulled out a notebook and started scribbling.

"See, if you look here, we've got two options, right? Only two. Either two people in our classroom of thirty have the same birthday . . . or they all have different birthdays. To make things simple, we'll ignore the February 29th conundrum for now. We'll stick with the standard 365. Now, we could break down all the different equations to figure out the probability of however many students sharing however many birthdays, but that's . . . well. That's the kind of thing I would do if I was having a slow day . . . week . . . month."

He scratched out his notes with a flourish.

Noa was nothing if not a showman.

He was a Kaiba, after all.

"So, instead, what we'll do is figure out the complement of what we want. Which is to say, the exact opposite. The best way to figure out complicated bullshit is to pick a different angle. So . . . what's the probability that nobody shares a birthday in a classroom of thirty? About 29 percent."

"Wait, wait, wait," Ryo said, holding up a hand. "How did you . . . ?"

"Okay, okay. So. First student. This one here. We don't have to worry about that one. They can have whatever birthday they want. The probability they'll have a relevant birthday for this equation is 365 out of 365. A hundred percent. The second student, though, this dude here, only has 364 possible birthdays if he's gonna suit our hypothetical. So that's 364 out of 365."

So Noa went writing down more and more equations, solving them so quickly that Ryo was convinced that he'd definitely given this speech before. It was practiced, smooth, confident, and utterly incomprehensible.

He may as well have been speaking backwards.

"We want x here. And x is 1 minus y. What we did here is figured out y. Again, 29 percent . . . ish. Take that away from 1 and you get 0.71. 71 percent chance that two or more students in a classroom of thirty will share a birthday with each other."

Ryo looked over the numbers in Noa's little notebook and wondered if this was how normal people felt when he discussed the rules to Monster World.