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What is the difference between "Tapchem": and "Beneichem?
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Oznayim la'Torah: Based on the Midrash Rabah, "Tapchem" refers to the children under bar-Mitzvah, none of whom died in the desert as a result of the decree, and "B'neichem" to those who were over bar-Mitzvah who did not participate in the sin. 1
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Oznayim la'Torah (Ibid.) Whereas the basic pertained to those who were twenty, whether thy sinned or not - and incorporated those over Bar-Mitzvah who participated in the sin.