Thirty parcels were delivered to a building that houses seven households, and every one of the parcels went to one of those seven. Which of the following must be true?

If every household had received four parcels or fewer, the seven together could account for at most 28 parcels, which is short of 30. Some household must therefore have received five or more. A household could have received none at all, so the first option fails; a split such as 6, 6, 6, 6, 6, 0, 0 leaves nobody with exactly five and gives several households more than five, so the second and fourth fail too.