For those that are to be redeemed It is probable, in the exchange they began with the eldest of the firstborn, and so downward, so that those were to be redeemed who were the two hundred and seventy- three youngest of them.
Related Commentaries of Numbers 3:46
Numbers 3:46
46 And for those that are to be redeemed of the two hundred and threescore and thirteen of the firstborn of the children of Israel, which are more than the Levites;