Mock exam F2 Q3 - Labour Efficiency Variance

Hello, I was reviewing my answers for the Revision mock exam and I sumbled into the following question : A company has a budgeted lab. cost of $ 180K for the production of 30K units per month. Each unit is budgeted to take 3h of labour. Actual labour cost was $ 160K for 28K units with 85K hours worked. What is the labour efficiency variance? My calculation was : Act h. worked 85K vs. h. for the actual production ( 28K x 3h = 84K) @ std. labour cost p.u. ( 180K/30K/3h=2$) and this should give us 2000 Adverse ( more h worked than the ones for the standard production) But the answer in the mock exam is 2000 F. Did I miss to include something in my calculation that would get me to 2K F? Many thanks. Cristina