| 摘要 |
A teaching aid for elementary arithmetic consists of transparent grids marked with ten squares in two rows, each square being marked with dots signifying numerals between zero and ten, zero and nine, or one and ten, opaque grids marked in the same way and on which the transparent grids can be superimposed, and a calculating ruler divided into two halves with ten blank squares in each. In addition to the dots, which can be in the same or different colours, the squares can be printed with the symbols for the numerals and their names.
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