(b) A directs B to sell A' s estate B, on looking over the estate before selling it, finds a mine on the estate which is unknown to A. B informs A that he wishes to buy the estate for himself, but conceals the discovery of the mine. A allows to buy in ignorance of the existence of the mine. A, on discovering that B knew of the mine at the time he bought the estate, may either repudiate or adopt the sale at his option.