Donna files as a head of household in 2013 and has taxable income of $90,000, including the

Question:

Donna files as a head of household in 2013 and has taxable income of $90,000, including the sale of a stock held as an investment for two years at a gain of $20,000. Only one asset was sold during the year and Donna does not have any capital loss carryovers.
a. What is the amount of Donna's tax liability?
b. What is the amount of Donna's tax liability if the stock is held for 11 months?