Assume that a company's planned level of activity was 3,500 units and its actual level of activity was 4,000 units. The spending variance for one of its mixed expenses was $900 favorable and its activity variance was $200 unfavorable. The planned amount of the fixed portion of this mixed expense was $10,000. What is the cost formula for the variable portion of this mixed expense? Multiple Choice $0.40 per unit $0.50 per unit $0.60 per unit $0.70 per unit