> The Quran does not provide specific guidelines on which types of wealth are taxable under the zakat, nor does it specify percentages to be given. However, it clearly indicates to donate the "surplus" of one's income. But the customary practice in the Islamic world has been that the amount of zakat paid on capital assets (e.g. money) is 2.5% (1⁄40).

you mean 2.5% tax rate was something made up that everyone happened to agree on and somehow works