How do proportional, progressive and regressive taxes meet the criteria of simplicity and equity?

A proportional tax is a fixed tax. This means that the tax base is constant despite an increase and decrease in tax. On the other hand, a progressive tax is a variable tax, in that, when the tax rate increases, the taxable amount also increase. A regressive tax is inversely proportional such that when tax rate decreases, the taxable amount increases. In doing this, all these taxes meet the criteria of simplicity and equity.