MacKenzie, Norman, 1921-2013 Open learning : Systems and problems in post-secondary education / Norman Mackenzie, Richmond Postgate, John Scupham. - Madrid, Esp. : UNESCO, 1975. - 498 p. ISBN: 9231013262 (softbound) 9231013327 (hardbound) No. de Clasificación Universal Decimal: 374 (100) =20 MAC