index.html
<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>My JavaScript</title>
</head>
<body>
<script src="main.js"></script>
</body>
</html>main.js
'use strict';
const english = Number(prompt('English'));
const math = Number(prompt('Math?'));
if(!(english >= 80 && math >= 80)){
console.log(`A`);
}else{
console.log(`B`);
}