The Robbie Corporation has a standard costing system in which variable manufacturing overhead is assigned to production on the basis of standard machine-hours. The following data are available for November Actual variable manufacturing overhead cost incurred $11,310 Actual machine-hours worked Variable overhead rate variance: Total variable overhead spending variance: ,600 hours S 1,710U S 2,310U The variable overhead efficiency variance for November is (YOU MUST SHOW YOUR WORK