4.4

function average(array) {
  function plus(a, b) { return a + b; }
  return array.reduce(plus) / array.length;
}
 
 var century = people(ancestry,function(person){
   return Math.ceil(person.died / 100);
 });

for(var cent in century){
  var ages = century[cent].map(function(person) {
    return person.died - person.born;
  });

  console.log(cent + ": " + average(ages));
}