Why is the zakat rate 2.5% according to Sunnis, what is it according to Shiites, and what is the proof of each?

The Zakat rate for gold and silver is 2.5% (a quarter of a tenth), and this is agreed upon by both Sunnis and Shiites, and it is supported by evidence from the Sunnah. The scholars have unanimously agreed that the Zakat on two hundred dirhams is five dirhams, and that if gold reaches twenty mithqals and its value is two hundred dirhams, then Zakat is due on it.