1a. A 68-year-old taxpayer has $20,000 in Social Security income and no other taxable or tax-free income. How much of the Social Security income must the taxpayer include in taxable income?

1b. A 68-year-old taxpayer has $20,000 in Social Security income and has significant other taxable retirement income. What is the maximum percentage of Social Security that the taxpayer might be required to include in taxable income?