| 摘要 |
<p>A device (2) for teaching addition or subtraction comprising a portion (10) having indicia (6) marked thereon, said indicia (6) representing numbers, means (8) adapted for movement relative and adjacent to the portion (10) and which can be selectively aligned with any one of the indicia (6), and means (24, 26) for indicating the direction in which the movement means (8) should be moved relative to the portion (10) so as to perform an addition or subtraction operation involving at least one of the numbers.</p> |
