Fifty college teachers are surveyed as to their possession of colour TV, VCR and tape recorder. Of them, 22 own colour TV, 15 own VCR and 14 own tape recorders. Nine of these college teachers own exactly two items out of colour TV, VCR and tape recorders; and one college teachers owns all three. Then how many of the 50 college teachers own none of three, colour TV, VCR or tape recorder?