Question 718160
For 9 months the loss was 600 per month 9(-600).
For the next 3 months the gain was 1,200 per month 3(1,200).
To find the average over 12 months you need to divide by 12 
[9(-600)+3(1,200)]/12
(-5,400+3,600)/12
-1,800/12=$-1,500 LOSS PER AVERAGE MONTH.
HOPE THIS HELPS EXPLAIN THE PROCESS.