Evidence for infants being able to "add" and "subtract" from Wynn (1992) would be their dishabituation at seeing:

Question

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Wynn (1992) found that infants showed surprise (measured by longer looking times) when the outcome of a simple addition or subtraction did not match what should logically occur. In the study, infants were shown scenarios where objects were added or removed from behind a screen. The correct scenario, C, where infants saw one toy mouse after two were placed behind a screen, would be surprising if they were expecting to see two mice, indicating an understanding of subtraction.

Evidence for Infants Being Able to "Add" and "Subtract" from Wynn