Sheila, a single taxpayer, is a retired computer executive with a taxable income of $90,000 in the current year. She receives $30,000 per year in tax-exempt municipal bond interest. Adam and Tanya are married and have no children. Adam and Tanya's $90,000 taxable income is comprised solely of wages they earn from their jobs. Calculate and compare the amount of tax Sheila pays with Adam and Tanya's tax. How well does the ability-to-pay concept work in this situation?