havent been able to figure out d. i already asked the question once but someone gave me the same answer as c. please explain

Assume the rate of interest is 8%. How much would you need to set aside to provide each of the following? a. $1 billion at the end of each year in perpetulty. (Enter your answer in billions rounded to 1 decimal place.) b. A perpetuity that pays $1 billion at the end of the first year and that grows at 4% a year. (Enter your answer in billions rounded to 1 decimal place.) c. $1 billion at the end of each year for 20 years. (Do not round intermediate calculations. Enter your answer in billions rounded to 3 decimal places.) d. $1 billion a year spread evenly over 20 years. (Do not round intermediate calculations. Enter your answer in billions rounded to 3 decimal places.) Answer is complete but not entirely correct