His fruit snacks will last 4 days. This is because Derek only has 2 boxes of fruit snacks which each have 12 fruit snacks in them. If he eats half of the box in one day, (12 divided by 2 equals 6) then he only has half to eat the next day so that equals two. Since there is two boxes with the same amount of snacks, you just have to add two and two which equals 4.
