If don loses 16 pounds,9 pounds, and 11 pounds during the first three months of his diet, how much will he have to lose during the fourth month so that he will have an average weight loss of 12 pounds per month?

If don want to lose weight 12 pounds per month in average for 4 months, then the total weight loss would be:

total weight loss= average weight loss * duration
total weight loss= 12 pound/month * 4 month= 48 pounds

In the third month don already loses: 16 pounds + 9 pounds+ 11 pounds = 36 pounds
Then the fourth month don need to lose: 48 pounds - 36 pounds = 12 pounds