7. A teaching device comprising in combination: a text book; a text key encoded on a surface of the text book; means for automatically sensing an encoded text key and generating in response thereto a digital text signal indicative of a text key number; means, responsive to the text signal, for automatically performing an arithmetic calculation on the text signal to generate a unique set of digital answer key signals indicative of a sequence of correct answers, at least some of said correct answers corresponding to respective questions associated with said text key; means for registering a manually generated answer signal indicative of a proposed answer to a selected one of said questions; and means for automatically comparing the manually generated answer signal with the set of answer key signals to determine whether the proposed answer corresponds to the respective correct answer.
7. A teaching device comprising in combination: a text book; a text key encoded on a surface of the text book; means for automatically sensing an encoded text key and generating in response thereto a digital text signal indicative of a text key number; means, responsive to the text signal, for automatically performing an arithmetic calculation on the text signal to generate a unique set of digital answer key signals indicative of a sequence of correct answers, at least some of said correct answers corresponding to respective questions associated with said text key; means for registering a manually generated answer signal indicative of a proposed answer to a selected one of said questions; and means for automatically comparing the manually generated answer signal with the set of answer key signals to determine whether the proposed answer corresponds to the respective correct answer. 12. The invention of claim 7 wherein the means for performing an arithmetic calculation comprises a pseudo-random number generator and means for supplying said text signal to the pseudo-random number generator as a seed to generate the set of answer key signals.