| 摘要 |
An arithmetic teaching aid with a front and a back sheet of the same size and shape in superposed alignment. A spacing lug is disposed between the sheets at each of four corners thereof. A large disc and two series of smaller discs, each series comprising three smaller discs having equal diameters are rotatively disposed between the sheets and along a side edge of the sheets. The front sheet has openings through which numerical indicia on the discs is visible.
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