A reversible arithmetic teaching device. In one embodiment, the device comprises a receptacle having on both sides a horizontal row of numbers with a corresponding arithmetic function below which is a window through which a reference number and arithmetically corresponding answers are visible. A slidable card is inserted in the receptacle. The card has on its first side a vertical column of numbers and an array of rows of comprising corresponding numerical answers. One number at a time on each vertical column is displayed through the window. Indicating means permit the indication of a selected arithmetic relationship between any one number in the column of numbers located at one end of the slidable card and any one of the horizontal row of numbers on the receptacle such that, upon applying the arithmetic designation with respect to the two cooperating numbers, a corresponding answer is displayed through the window.