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US2877566A
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Educational toy with problem solution indication
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Educational problem and answer device
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Educational device with interfittable problem and answer members
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Educational device presenting pictorial and numerical problems
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Educational device teaching reading and arithmetic
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Problem teaching device using answer blocks
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Self-releasing trap nest
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Band-stamp.
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US778790A
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Calculator.
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Tabular reference device.
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CA501789A
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Educational device presenting pictorial and numerical problems
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