Under the 9-9-9 plan the lowest quintile would pay 18% for every dollar earned and spent. They would pay 9% on every dollar earned. Currently they are paying 10% on every dollar earned plus 15% for payroll taxes (fica and medicare) 18<25.
And on the revenue side the fed gets to capture at least 9% from those involved in the underground economy when they purchase stuff.
For the fairness.