Discovery BoxNumbers 18:15-16 
Context18:15 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).
1 tn The construction uses the infinitive absolute and the imperfect tense of the verb “to redeem” in order to stress the point – they were to be redeemed. N. H. Snaith suggests that the verb means to get by payment what was not originally yours, whereas the other root גָאַל (ga’al) means to get back what was originally yours (Leviticus and Numbers [NCB], 268).
