sl.js

let sum = function (x, y, z) {
  return new Promise((resolve, reject) => {
    let sum = x + y + z;
    resolve(sum);
  });
};

let average = function (total) {
  return new Promise((resolve, reject) => {
    let average = total / 3;
    resolve(average);
  });
};
let percentage = function (average_value) {
  return new Promise((resolve, reject) => {
    let percentage = Math.floor(average_value) + "%";
    resolve(percentage);
  });
};

sum(20, 40, 40)      
  .then((value) => {
    return average(value); 
  })
  .then((value) => {
    return percentage(value);
  })
  .then((value) => {
    console.log(value);    // 33%
  });