Owl Vision Corporation (OVC) is a North Carolina corporation engaged in the manufacture and sale of contact lenses and other optical equipment. The company handles its export sales through sales branches in Belgium and Singapore. The average tax book value of OVC's assets for the year was $100 million, of which $80 million generated U.S. source income and $20 million generated foreign source income. OVC's total interest expense was $10 million.

What amount of the interest expense will be apportioned to foreign source income under the tax book value method?