Please explain how to solve

Rickett Corporation had a favorable direct-labor efficiency variance of $2,100 for the period just ended. The actual wage rate was $0.50 more than the standard rate of $10.00. If the company's standard hours allowed for actual production totaled 9,700 , how many hours did the firm actually work? Multiple Choice 9.490. 9.500 9.900 9.910. None of the answers is correct