The only two things that matter here are the number of grandchildren who need new coats, and the number of coats he buys for those grandchildren.
He will be buying new winter coats. How does he know how many to buy? He will buy the number of coats needed for the grandchildren who outgrew their winter coats. The number of winter coats he buys is a function of the number of grandchildren who need new coats.
The number of winter coats he buys depends on the number of grandchildren who outgrew their coats. That means that the number of grandchildren who outgrew their coats is the independent variable. The dependent variable is the number of coats he buys because it depends on the number of grandchildren who outgrew their winter coats.
Answer: o is the independent variable and n is the dependent variable
