underscore-2.1.js

var students = [
  { name: "John", grades: [7.0, 6.9, 8.0], class: "A" },
  { name: "Paul", grades: [5.0, 8.7, 3.7], class: "A" },
  { name: "Luke", grades: [9.0, 8.2, 6.2], class: "B" },
  { name: "Mark", grades: [6.0, 9.0, 8.4], class: "B" }
];

var sum = function (numbers) { return _.reduce(numbers, function (sum, n) { return sum + n }, 0) }
var average = function (numbers) { return sum(numbers) / numbers.length }
var isApproved = function (student) { return average(student.grades) >= 7 }