Answer:
15 boxes
Step-by-step explanation:
Because 71 divided by 5 equals to 14.2, you know that you need at least 14 boxes. However, you cannot have 14.2 boxes, because that implies that you have a fifth of a box, which cannot happen. But because each box can only hold up to 5 cards max, you need to get a 15th box for the last card.