Longview Hospital performs blood tests in its laboratory. The following standards have been set for each blood test performed: Picture During May, the laboratory performed 1,500 blood tests. On May 1 there were no direct materials (plates) on hand; after a plate is used for a blood test it is discarded. Variable overhead is assigned to blood tests on the basis of standard direct labor-hours. The following events occurred during May: 3,600 plates were purchased for $9,540 3,200 plates were used for blood tests 340 actual direct labor-hours were worked at a cost of $5,550 The direct materials purchases variance is computed when the materials are purchased. The variable overhead efficiency variance for May is $350 F $350 U $280 U $280 F