i Have json like that but bigger, and i need average of any DB,TV,... there is not only 1 student but more here is only exam how to in javascript

[
  {
    "Petr Prochazka": {
        "DB": [
            "5",
            "4",
            "2",
            "4",
            "4",
            "2"
        ],
        "TV": [
            "4",
            "1",
            "2",
            "2",
            "4",
            "5",
            "5",
            "5"
        ],