Calculate the present value, discounted at 10 percent, of receiving: (a) $800 at the end of year 4; (b) $200 at the end of year 3 and $300 at the end of year 5; (c) $500 at the end of year 4 and $300 at the end of year 6; and (d) $500 a year for the next 10 years.