#!/bin/awk -f
# ./moyenne_groupe fichier <nom groupe>

BEGIN {
    FS = ","
    GROUP = ARGV[2]
    ARGV[2] = ""

    NOTES = 0
    SOMME = 0
}

$3 == GROUP {
    SOMME += $4
    NOTES += 1
}

END {
    print SOMME / NOTES
}