Please explain how I calculate the predetermined overhead rate to complete this problem.

A firm has applies variable overhead with a standard rate of $20 per machine hour (assume there is no fixed overhead for this problem). The firm's actual finished goods units were 10,000. The standard quantity is 2 machine hours per finished good unit. The firm's actual machine hours were 21,000, with an actual cost per machine hour of $22. What is the firm's variable overhead quantity variance (round final answer to nearest cent if necessary)? a $22,000 favorable O b. $22.000 unfavorable O c. $20,000 unfavorable O d. $20,000 favorable