HP12C Rule of 78s Program enclosed Message #1 Posted by Gene Wright on 18 Sept 2000, 4:25 p.m.
Somewhere down below in the lists of posts to this messageboard, a request is made for a rule of 78s interest rebate program for the HP12C. The Rule of 78s is a method used by finance companies to determine the amount of unearned interest that is rebateable to a customer if they pay off a loan before its maturity. It is somewhat disadvantageous to the customer, as it is an accelerated method of recognizing interest earnings and hence lowers the rebate amount if a payoff occurs quite quickly.
Here is one that will run on it. It can easily be modified to run on any HP RPN programmable. Hope it helps.
Registers:
0  Original Unearned Interest Amount (INPUT)
1  Original Term of Loan (INPUT)
2  Remaining Term of Loan (Program Calculated)
3  Current Earned Interest Amount (Program Calculated)
4  Remaining Unearned Interest Amount (Program Calculated)
Instructions:
1) Enter Program
2) Press GTO 00
3) Enter original unearned interest amount and press STO 0
4) Enter original term of loan and press STO 1
5) Press R/S to display first months remaining unearned interest
6) Continue pressing R/S to display each consecutive month's unearned interest.
Example: John is paying for a car with a 36 month loan. The car costs $10,000 and his monthly payment is $313.36.
Step 1: The original unearned interest is ($313.36 x 36)  $10,000 or $1,281.09
Step 2: 1281.09 STO 0
Step 3: 36 STO 1
Step 4: Press R/S > See 1142.79 displayed.
Step 5: Press R/S > See 1079.30 displayed.
Step 6: Press R/S > See 1017.63 displayed.
If the John pays off the car balance after the third payment, he will need to pay the original amount of the loan ($10,000) + the original interest charges ($1,281.09) less his payments (3 x 313.36 or $940.08) less a rebate of $1,017.63. His payoff total is $9,323.38.
Program Listing:
Notes: 1) the divide key is listed as DIVIDE. 2) If instead of seeing the remaining unearned interest each time you want to see the amount of interest earned each period, replace step 27 with RCL 3. 3) When the calculator stops to display a result, you may do any calculation you wish as long as you do not change memories 05 or move the program's location. The stack is ignored on each loop.
01 RCL 1
02 RCL 1
03 1
04 +
05 x
06 STO 5
07 RCL 1
08 1
09 
10 STO 2
11 RCL 2
12 RCL 2
13 1
14 +
15 x
16 RCL 5
17 DIVIDE
18 RCL 0
19 x
20 RCL 4
21 X<>Y
22 STO 4
23 
24 STO 3
25 1
26 STO  2
27 RCL 4
28 R/S
29 GTO 11
