Bible: Num. 18:15-16
18:15 The firstborn of every womb which they present to the
Lord, whether human or animal, will be yours. Nevertheless, the firstborn sons you must redeem, 1 and the firstborn males of unclean animals you must redeem. 18:16 And those that must be redeemed you are to redeem when they are a month old, according to your estimation, for five shekels of silver according to the sanctuary shekel (which is twenty gerahs).
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