Consider a couple, both partners are ages of 68 and their status is married filing jointly for the 2021 tax year. Mary is permanently disabled for the entire year, while Tom is unemployed for part of the year. They receive income from the following sources:

Tom's salary: $16,500

Tom's unemployment compensation: $4,500

Mary's disability income: $6,200

Social Security income: $1,000

a. Calculate taxable income, assuming the disability insurance premium was paid entirely by Mary.

b. Calculate taxable income, assuming the disability insurance premium was paid entirely by Mary's employer.