Final answer:
A tax is proportional if its average rate remains the same regardless of the size of income.
Step-by-step explanation:
A tax is proportional if its average rate remains the same regardless of the size of income. In other words, everyone pays the same percentage of their income as tax. For example, if the tax rate is 10%, then both someone earning $10,000 and someone earning $100,000 would pay $1,000 and $10,000 in taxes, respectively. This type of tax is also known as a flat tax because the tax rate remains constant for all income levels.