F Company, which uses the high-low method, reported total costs of $24 per unit at its lowest activity level, when production equaled 10,000 units. When production doubled, at its highest activity...

F Company, which uses the high-low method, reported total costs of $24 per unit at its lowest activity level, when production equaled 10,000 units. When production doubled, at its highest activity level, the total cost per unit dropped to $15. Foster would estimate fixed costs as?