Please compute the additional payment. also please include the formula used in excel.Thank You!!

Auto Loan Payment Calculator
	Inputs
		Auto Loan Amount	$20,000
		Annual Interest Rate	8.50%
		Term of Loan in Years	3
		First Payment Date	2/1/18
		Last Payment Date	1/1/21
		Frequency of Payment	Monthly
	Summary
		Number of Payments	36
		Rate (per period)	0.708%
		Payment (per period)	$631.35
		Total Interest	$2,728.63
		Total Payments	$22,728.63
No.	Due Date	Payment Due	Additional Payment	Interest	Principal	Balance
						$20,000.00
1	2/1/18	631.35	0.00	141.67	489.68	19,510.32
2	3/1/18	631.35	0.00	138.20	493.15	19,017.17
3	4/1/18	631.35	0.00	134.70	496.65	18,520.52
4	5/1/18	631.35	0.00	131.19	500.16	18,020.36
5	6/1/18	631.35	0.00	127.64	503.71	17,516.65
6	7/1/18	631.35	0.00	124.08	507.27	17,009.38
7	8/1/18	631.35	0.00	120.48	510.87	16,498.50
8	9/1/18	631.35	0.00	116.86	514.49	15,984.01
9	10/1/18	631.35	0.00	113.22	518.13	15,465.88
10	11/1/18	631.35	0.00	109.55	521.80	14,944.08
11	12/1/18	631.35	0.00	105.85	525.50	14,418.58
12	1/1/19	631.35	0.00	102.13	529.22	13,889.36
13	2/1/19	631.35	0.00	98.38	532.97	13,356.39
14	3/1/19	631.35	0.00	94.61	536.74	12,819.65
15	4/1/19	631.35	0.00	90.81	540.54	12,279.11
16	5/1/19	631.35	0.00	86.98	544.37	11,734.74
17	6/1/19	631.35	0.00	83.12	548.23	11,186.51
18	7/1/19	631.35	0.00	79.24	552.11	10,634.40
19	8/1/19	631.35	0.00	75.33	556.02	10,078.38
20	9/1/19	631.35	0.00	71.39	559.96	9,518.42
21	10/1/19	631.35	0.00	67.42	563.93	8,954.48
22	11/1/19	631.35	0.00	63.43	567.92	8,386.56
23	12/1/19	631.35	0.00	59.40	571.95	7,814.61
24	1/1/20	631.35	0.00	55.35	576.00	7,238.61
25	2/1/20	631.35	0.00	51.27	580.08	6,658.53
26	3/1/20	631.35	0.00	47.16	584.19	6,074.34
27	4/1/20	631.35	0.00	43.03	588.32	5,486.02
28	5/1/20	631.35	0.00	38.86	592.49	4,893.53
29	6/1/20	631.35	0.00	34.66	596.69	4,296.84
30	7/1/20	631.35	0.00	30.44	600.91	3,695.93
31	8/1/20	631.35	0.00	26.18	605.17	3,090.76
32	9/1/20	631.35	0.00	21.89	609.46	2,481.30
33	10/1/20	631.35	0.00	17.58	613.77	1,867.53
34	11/1/20	631.35	0.00	13.23	618.12	1,249.40
35	12/1/20	631.35	0.00	8.85	622.50	626.90
36	1/1/21	631.34	0.00	4.44	626.90	0.00