This can be stated in active form. AT: "The people must buy them back when they become one month old" (See: [[rc://en/ta/man/translate/figs-activepassive]])
Most likely this refers only to firstborn humans that must be bought back and not to firstborn unclean animals.
# five shekels ... which equals twenty gerahs
If it is necessary to use modern weight units, here are two ways of doing it. AT: "five pieces of silver ... each of which equals ten grams" or "fifty grams of silver, using the standard weights that are used in the sanctuary" (See: [[rc://en/ta/man/translate/translate-bweight]])
# five shekels
A shekel is a unit of weight. What was weighed can be stated clearly. AT: "five shekels of silver" (See: [[rc://en/ta/man/translate/figs-explicit]])
There were shekels of different weights. This is the one that people had to use in the sanctuary of the sacred tent. It weighed twenty gerahs, which was about 11 grams. (See: [[rc://en/ta/man/translate/translate-bweight]])