4. Zach, a single taxpayer, is retired and received $10,000 of social security benefits this year. Zach had pension income of $28,000 (this does count and is modified AGI), which was the only other income item he had. a. What portion of the $10,000 social security benefits are taxable? b. What portion of the $10,000 social security benefits would be taxable if pension income were $19,000 instead of $28,000