Answer: 2400 hours
Step-by-step explanation: Labor efficiency variance can be computed using following formula :-
Labor efficiency variance = standard rate (standard hours - actual hours)
so, putting the values into equation we get :-
-$1215 = $8.10 ( 2,550 hours - actual hours)
-$150 = 2,550 hours - actual hours
actual hours = 2400 hours