Liang, an NRA, is sent to the United States by Fuller Brush Co., her foreign employer. She spends 50 days in the United States and earns $20,000 for a two-month period. This amount is attributable to 40 U.S. working days and 10 non-U.S. working days. Her employer does not have a U.S. trade or business and Liang spends no other time in the U.S. for the tax year. Liang's U.S.-source taxable income is how much? Explain your answer.