I am having a hard time trying to the following problem.  Any help,
web sties, etc. will be appreciated.

A firm currently offers terms of sale of 3/20, net 40.  What effect
will the following actions have on the implicit rate charged to
customers that pass up the cash discount?  State whether the implicit
rate will increase or decrease.
a) The terms are changed to 4/20, net 40.
b) The terms are changed to 3/30, net 40.
c. The terms are changed to 3/20, net 30.

Boobee -

Good to see you back at Google Answers!

The problems that you're having are over "prompt  payment discounts." 
Unfortunately most finance-oriented websites deal with discounted cash
flow - but not this key area in Accounts Payable accounting.

However, it's an application of discounted cash flow and is both "real
world" and astounding in financial impact.  The financial impact, as
you'll see, is so high because of the discounts offered for a short
period that a University of Connecticut website refers to them as
"free money."

Terms of sale are often referred to as "3/20, net 40" and it
translates this way:
full payment due in 40 days; 3% discount given for payment in 20 days

How can it be evaluated?  If a firm can quality-control the product or
service; get internal approval to pay; and cut the check within 20
days - it gets a 3% discount for paying 20 days before the deadline. 
In this case the discount is so large that compounding makes it worth
even more than a simple annualized 3% x 365/20 = 54.75%.  It is
calculated by multiplying 1.03 by itself 18.25 times = 1.715,
indicating an annualized interest rate of 71.5%.

If terms are changed to 4/20, net 40:
4% discount for payment 20 days early = 1.04^18.25 = 2.046 = annual
interest rate of 104.6%

If terms are changed to 3/30, net 40:
3% discount for payment 10 days early = 1.03^36.5 = 2.941 = annual
interest rate of 194.1%

If terms are changed to 3/20, net 30:
3% discount for payment 10 days early, which returns the same annual
rate as the previous calculation

These terms may seem extraordinary but it helps show the importance of
prompt Account Receivable policies for a firm.  Probably the
most-common commercial discounts are 1/10, net 30 or 2/10, net 30 -
but even the former uses an imputed rate of 20% per year or about what
consumers pay for expensive credit card debt.

A Google search strategy using:
"prompt payment discount" + explanation 
or 
"prompt payment discount" + "accounts payable"
or 
"prompt payment discount" + "accounts receivable"

returns a large number of government and commercial contract and
software sites. (You'll see how lucrative they are for both buyer and
seller.)

So I've had to rely on professional experience here.  If any part of
this answer is unclear, please let me know through a clarification
request before rating this answer.

Best regards,

Omnivorous-GA

Thanks Omnivorous-ga.  Good to be back -- you will probably be seeing
a lot of me in the next few weeks.  Thank you so much.  You made this
so clear and so easy.  Five GOLD stars for you!!!