Fixed Overheads-total variance

The budgeted fixed production overhead for H Ltd for the year 2012 is $48,000. Budgeted direct labour time for the year was 12,000 hours. The standard labour time for one unit of product was 3 hours.

During the year, the actual production was 3900 units and the actual number of hours worked was 11500 hours. The actual fixed overhead was $50000.

If want to find the total variance the below calculation whether is correct ?

my working:
OAR = $48000 / 12000 hrs = $4/hr x one unit use 3 hrs = $12 per unit ?

actual total cost $50000
compare with standard costs with actual production 3900 units x 3 hrs = 11700 hrs x $4=$46800
total variance $3,200 adverse ? ( can call this as over / under absorption ?)

Sir, I would like to know the standard cost per unit whether is $12 or $4 ?
I remember in your lecturer variance analysis for calculate total variance use $15 per unit not $3 per hour :
actual total cost 8900 units $134,074
standard cost for actual production 8900 units x $15=$133500
$574 adverse

please correct me the above. Thank you.