Here is a QUICK Answer:
Let's assume that we are talking about calendar years and we are
sitting there on January 1st asking this question. The answer depends
on whether the $100 payments are received on January 1, meaning today,
or whether the money is received on December 31st.
The payment received on January 1 of each year is a "beginning annuity."
The payment received on December 31 of each year is an "ending annuity."
Most calculators assume that you are calculating for an ending annuity.
A. 8% 10 $671.01
B. 8% 20 $981.81
C. 4% 10 $811.09
D. 4% 20 $1,359.03
A. 8% 10 $724.69
B. 8% 20 $1,060.36
C. 4% 10 $843.53
D. 4% 20 $1,413.39
Here is a website from Duke University that explains some of the math:
Frankly, I just plugged the variables (time, rate, and payment) into
my trusty Hewlett-Packard 12C calculator.
If you need help making the formulas work from the Duke site, please
let me know by clicking the CLARIFICATION button.