そろばんの四則演算アプリ

下のボタンを押すと、20個のそろばんの四則演算問題が自動的に作成されます。
問題

<!DOCTYPE html>
<html>
<head>
	<meta charset="UTF-8">
	<title>そろばんの四則演算アプリ</title>
</head>
<body>
	<h1>そろばんの四則演算アプリ</h1>
	<p>下のボタンを押すと、20個のそろばんの四則演算問題が自動的に作成されます。</p>
	<button onclick="createQuestions()">問題を作成する</button>
	<h2>問題</h2>
	<div id="questions"></div>
	<script>
		function createQuestions() {
			var questionsDiv = document.getElementById("questions");
			questionsDiv.innerHTML = "";
			for (var i = 0; i < 20; i++) {
				var a = Math.floor(Math.random() * 100);
				var b = Math.floor(Math.random() * 100);
				var c = Math.floor(Math.random() * 100);
				var operator1 = "+";
				var operator2 = "-";
				var operator3 = "×";
				var operator4 = "÷";
				var operator = "";
				var answer;
				switch (Math.floor(Math.random() * 4) + 1) {
					case 1:
						operator = operator1;
						answer = a + b + c;
						break;
					case 2:
						operator = operator2;
						answer = a - b - c;
						break;
					case 3:
						operator = operator3;
						answer = a * b * c;
						break;
					case 4:
						operator = operator4;
						answer = a / b / c;
						break;
				}
				var questionText = (i + 1) + ". " + a + " " + operator + " " + b + " " + operator + " " + c + " = ?";
				var questionDiv = document.createElement("div");
				questionDiv.textContent = questionText;
				questionsDiv.appendChild(questionDiv);
			}
		}
	</script>
</body>
</html>