This can be stated in active form. Alternate translation: "The people must buy them back when they become one month old" (See: [[rc://en/ta/man/translate/figs-activepassive]])
If it is necessary to use modern weight units, here are two ways of doing it. Alternate translation: "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]])
A shekel is a unit of weight. What was weighed can be stated clearly. Alternate translation: "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]])