Read Numbers 18:16 Using Other Translations
And those that are to be redeemed from a month old shalt thou redeem, according to thine estimation, for the money of five shekels, after the shekel of the sanctuary, which is twenty gerahs.
And their redemption price (at a month old you shall redeem them) you shall fix at five shekels in silver, according to the shekel of the sanctuary, which is twenty gerahs.
Redeem them when they are one month old. The redemption price is five pieces of silver (as measured by the weight of the sanctuary shekel, which equals twenty gerahs).