Cheyenne Corporation is a U.S. corporation engaged in the manufacture and sale of mining equipment. The company handles its export sales through sales branches in Canada and Mexico. The average tax book value of Cheyenne's assets for the year was $200 million, of which $100 million generated U.S. source income and $100 million generated foreign source income. Cheyenne's total interest expense for the year was $30 million. What amount of interest expense can Cheyenne apportion against its foreign source gross income for foreign tax credit purposes, assuming there is no limitation on the interest expense deduction?