The average is the sum of the observations divided by the count.
So the sum of the observations is the count times the average.
So the 9 teachers together make 9(1000)=$9000 per week. The 20 staff (9 teachers + 11 aps) makes 20(1275)=25500.
So the 11 aps must make 25500-9000=16500
So the mean ap salary is 16500/11 =$1500 per week
Clearly management is where it's at.
Check: (1500*11+1000*9)/20 = 1275 good
Choice B