2.3 Franklin is a single taxpayer, 63 years old, and a bona fide resident of Puerto Rico with no dependents. Franklin received a Form SSA-1099, Social Security benefits, showing $26,700 in box 3. He did not receive any other income. Use Publication 4696 Social Security Benefits section as your reference materials. Is any part of his Social Security benefits subject to U.S. income tax?

Santiago is single, 70 years old and received Form SSA-1099 for 2023, which shows benefits of $19,000 in box 5. He also received the following income during 2023: 

Taxable pension from the University of Puerto Rico of $37,000

Salary of $5,000 from a company in Puerto Rico

Taxable IRA distribution of $4,000 from a bank in Puerto Rico

 What amount of his Social Security benefit is taxable to the United States?