I don't understand this PLEASE help! How would costs change if you could add 100 pounds of nitrogen at 25 cents a pound and get approximately the same yield response?

Suppose your rich uncle gave you $50,000, which you plan to use for graduate school. you will make the investment now, you expect to earn an annual return of 6%, and you will make 4 equal annual withdrawals, beginning 1 year from today. under these conditions, how large would each withdrawal be so there would be no funds remaining in the account after the 4th withdraw?