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ID110-Changing an Exposure
Question: Can I change my exposure if I put up too few or too many Jokers?
Answer: If a player inadvertently exposes the wrong number of tiles or exposes too many jokers, they can add to or take away from the current exposure as long as they have not discarded. After a player has discarded, exposures on their rack cannot be changed.
Source: Mah Jongg Made Easy (2020 page 15 para.2)