Javascript calcular média com reduce

[Cordas]

Eu tenho uma dúvida de como fazer a média. Supomos que tenho esse exercício:

 

<p>1) Click to sort the numbers you tipped</p>
<button onclick="myFunction()">Sort</button>
<p>2) Click to sort the numbers from largest to smallest</p>
<button onclick="myFunction2()">Sort</button>
<p>3) Click to see the Average</p>
<button onclick="myFunction3()">Sort</button>
<p>4) Click to see the Highest</p>
<button onclick="myFunction4()">Sort</button>
<p>5) Click to see the Lowest</p>
<button onclick="myFunction5()">Sort</button>
<div id="output"></div>
<script>
var gpas = [];
var theGPA = "";
//
while (theGPA != "exit")
{
theGPA = prompt("Enter a GPA or exit to Stop");
if(theGPA != "exit"){
gpas.push(theGPA);
}
}
//
var out = "";
for(var x = 0; x < gpas.length; x++)
{
out += gpas[x];
out += " ";
}
document.getElementById('output').innerHTML = "Tipped numbers: <br>" + out;
//function sort
function myFunction(){
  gpas.sort();
  document.getElementById('output').innerHTML = "Sorted numbers: <br>" + gpas;
}
//function sort largest to smallest
function myFunction2(){
  gpas.sort(function(a,b){
    return b - a;
  });
document.getElementById('output').innerHTML = "From the largest to smallest: <br>" + gpas;
}
/*
var total = 0;
for(var x = 0; x < gpas.length; x++){
  total += gpas[x];
var avg = total/gpas.length;
}*/
function myFunction3(){
let total = gpas.reduce((a, b) => a + b);
let avg = total/gpas.length;
document.getElementById('output').innerHTML = "The Average is: <br>" + avg;
}
function myFunction4(){
  gpas.sort(function(a,b){
    return b - a;
  });
  document.getElementById('output').innerHTML = "Highest: <br>" + gpas[0];
}
function myFunction5(){
  gpas.sort(function(a,b){
    return a - b;
  });
  document.getElementById('output').innerHTML = "Lowest: <br>" + gpas[0];
}
</script>

Como eu faço para realizar o cálculo da média. Com o reduce não tô conseguindo ou estou implementando de maneira incorreta.