Functions call functions

Functions can call other functions:

function gross(net) {
  let vat = 20;
  let tax = net * vat / 100;
  return net + tax;
}

function addWithVat(netX, netY) {
  let grossX = gross(netX);
  let grossY = gross(netY);
  return grossX + grossY;
}

addWithVat takes two net amounts and returns the sum of the gross amounts. gross calculates the gross amount from a net amount. Thereby a value added tax of 20% is assumed. Instead of calculating the gross amount itself, addWithVat use the function brutto. This has two advantages: The code is easier to read and the gross calculation is performed centrally in one place.

Write a function sum that takes an array of numbers and returns the sum of these numbers. Write a function mean that takes an array of numbers and returns the average of these numbers. The mean function should use the sum function.