a. The popcorn bags decreased by 20%
The discount is

Thus the new bag hold 15-3=12 cups of popcorn.
The answer is 12.
b. Let the original price be x.
Price after discount is 12.
Thus,

Thus the original price is 16.
c.
Let the original population be x.
It is increased by 25%
Thus,

Thus the populationlast year was 5280.