JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text">

CSS

from random import shuffle
from copy import deepcopy

boxes = [ [100,100], [100,1]]
other_note_was_1=0
other_note_was_100=0

# Perform the experiment a million times
for _ in range(1000000):
    shuffle(boxes) # Shake!
    shuffle(boxes[0]) # Shake!
    shuffle(boxes[1]) # Shake!
    box = boxes[0] # Grab one of the two boxes
    note = box[0] # Grab one of the two notes in the box
    if note==100:
        # If the note is 100 observe the other note and keep count of its denomination.
        if box[1] == 1:
            other_note_was_1 +=1
        else:
            other_note_was_100 += 1
    else:
        # Challenge doesn't tell us what to do here.
        pass

probability = float(other_note_was_100) / (other_note_was_100 + other_note_was_1)
print(probability)

JavaScript

function shuffle(array) {
  let currentIndex = array.length,  randomIndex;

  // While there remain elements to shuffle.
  while (currentIndex != 0) {

    // Pick a remaining element.
    randomIndex = Math.floor(Math.random() * currentIndex);
    currentIndex--;

    // And swap it with the current element.
    [array[currentIndex], array[randomIndex]] = [
      array[randomIndex], array[currentIndex]];
  }

  return array;
}

let boxes = [ [100,100], [100,1]]
let other_note_was_1=0;
let other_note_was_100=0;

for (let i = 0; i < 1000000; i++) {
    shuffle(boxes) //# Shake!
    shuffle(boxes[0]); //# Shake!
    shuffle(boxes[1]); //# Shake!
    box = boxes[0]; //# Grab one of the two boxes
    note = box[0]; //# Grab one of the two notes in the box
    
    if (note==100) {
        // # If the note is 100 observe the other note and keep count of its denomination.
        if (box[1] == 1) {
            other_note_was_1 +=1;
        } else {
            other_note_was_100 += 1;
        }
    } else {
        // # Challenge doesn't tell us what to do here.
        continue;
    }
}

probability = (other_note_was_100) / (other_note_was_100 + other_note_was_1)
console.log(probability);
$('input').val(probability);