Shekel: A Weight, Equal to Twenty Gerahs

Everyone who crosses over to those counted must pay a half shekel, according to the sanctuary shekel, which weighs twenty gerahs. This half shekel is an offering to the LORD.

…you are to collect five shekels for each one, according to the sanctuary shekel of twenty gerahs.

The shekel will consist of twenty gerahs. Twenty shekels plus twenty-five shekels plus fifteen shekels will equal one mina.