Question

I got the answer to previous questions via the table, but what is the formula for interest & principle

Answer 1

Interest rate = 6.5%/12 (as monthly payments)

Thus formula for interest = 6.5%/12*beginning loan balance due. Thus in month 1 formula for interest = 6.5%/12*18900 = 102.38

Principle = Monthly payments - interest

For month 1 principal = 448.21 - 102.38 = 345.84

In the 2nd month starting loan balance = loan balance at the start of month 1 - principal paid in month 1

= 18900-345.84

= 18554.16

Thus interest in month 2 = 6.5%/12*18554.16 = 100.50

Principal = 448.21-100.50 = $347.71

In the 3rd month starting loan balance = loan balance at the start of  month 2 - principal paid in month 2

= 18554.16 - 347.71 = $18206.45

The same cycle will continue till the 48th month.

Period	Payment	Interest	Principle	Balance
1	448.21	102.38	345.84	18,554.16
2	448.21	100.50	347.71	18,206.45
3	448.21	98.62	349.59	17,856.86
4	448.21	96.72	351.49	17,505.37
5	448.21	94.82	353.39	17,151.98
6	448.21	92.91	355.31	16,796.67
7	448.21	90.98	357.23	16,439.44
8	448.21	89.05	359.17	16,080.27
9	448.21	87.10	361.11	15,719.16
10	448.21	85.15	363.07	15,356.10
11	448.21	83.18	365.03	14,991.06
12	448.21	81.20	367.01	14,624.05
13	448.21	79.21	369.00	14,255.05
14	448.21	77.21	371.00	13,884.06
15	448.21	75.21	373.01	13,511.05
16	448.21	73.18	375.03	13,136.02
17	448.21	71.15	377.06	12,758.96
18	448.21	69.11	379.10	12,379.86
19	448.21	67.06	381.16	11,998.70
20	448.21	64.99	383.22	11,615.48
21	448.21	62.92	385.30	11,230.19
22	448.21	60.83	387.38	10,842.81
23	448.21	58.73	389.48	10,453.33
24	448.21	56.62	391.59	10,061.74
25	448.21	54.50	393.71	9,668.02
26	448.21	52.37	395.84	9,272.18
27	448.21	50.22	397.99	8,874.19
28	448.21	48.07	400.14	8,474.05
29	448.21	45.90	402.31	8,071.74
30	448.21	43.72	404.49	7,667.25
31	448.21	41.53	406.68	7,260.56
32	448.21	39.33	408.88	6,851.68
33	448.21	37.11	411.10	6,440.58
34	448.21	34.89	413.33	6,027.25
35	448.21	32.65	415.56	5,611.69
36	448.21	30.40	417.82	5,193.87
37	448.21	28.13	420.08	4,773.79
38	448.21	25.86	422.35	4,351.44
39	448.21	23.57	424.64	3,926.80
40	448.21	21.27	426.94	3,499.85
41	448.21	18.96	429.26	3,070.60
42	448.21	16.63	431.58	2,639.02
43	448.21	14.29	433.92	2,205.10
44	448.21	11.94	436.27	1,768.83
45	448.21	9.58	438.63	1,330.20
46	448.21	7.21	441.01	889.19
47	448.21	4.82	443.40	445.80
48	448.21	2.41	445.80	-              0.00