Calculate the present value of each of the following income streams using a 5% discount rate:
a. An income stream of $1,000 per year for 20 years.
b. An income stream of $1,000 per year for years 1-10 followed by an income stream of $500 per year for years 11-20.
c. A net income stream of -$1,000 per year for years 1-3 followed by an income stream of $2,000 per year for years 4-10.
d. A perpetual income stream of $1,000 per year.
e. A perpetual income stream of $10,000 per year.