cardTypes solution

cardTypes.js

let cardTypes = [4, 7, 5, 11, 15];

function cardPackets(cardTypes) {
  let packets = [2, 3]; //Assume two packets will require fewer additional cards, but check three packets, as well
  let addlCardsSums = [0, 0]; //Store the sum of additional cards needed, per packet quantity (2 or 3, for now)
  let addlCards = Array(cardTypes.length).fill(0); //Create a base array of [0,0, ..., 0] to capture add'l cards needed by type

  packets.forEach((packet, i) => { //Iterate through 2 and 3 packets versions
    cardTypes.map((count, j) => { //Map through cardTypes and increment until each type is divisible by the number of packets
      while (count % packets[i] !== 0) {
        count++;
        addlCards[j]++;
      }
      return count;
    });

    addlCardsSums[i] = addlCards.reduce((sum, num) => sum += num); //Sum up the final tally of additional needed cards, for this number of packets
  })

  let cards = Math.min(...addlCardsSums); //Find the lowest number of additional needed cards
  let index = addlCardsSums.indexOf(cards); //Find the correlating index in the packets array
  return `${cards} additional cards required for ${packets[index]} packets.`; //Output the result!
}

cardPackets(cardTypes);