please explain the math behind questions 18-21

Use the following to answer questions 18-21: Hurricane, Inc. manufactures hurricane lamps. Selected data regarding standard costs follow: Direct materials per unit 3.5 pounds at $5.00 per pound Direct labor per unit 3 hours at $9.50 per hour Selected actual results for the month of August are shown below: Units produced Direct materials purchased 5,500 20,000 pounds at a total cost of $104,000 19,400 pounds 16,800 hours at a total cost of $163,800 Direct materials used Direct labor used 18. The direct labor price variance is: A) $2,850U B) $2,925F C $4,125F D$4,2000 19. The direct labor usage variance is: A) $2.850U B) $2,925F C) $4,125F D) $4,200U 20. The direct materials price variance is: A) $3,750F B) $3,850F C) $3,880U $4,000U 21. The direct materials usage variance is: A) $_7500 B) $ 780F C) $3,000U D) $3,750F