how to calculate the seasonal index and what is the equation? Trendline? seasonal ratio? forecast? need equations so I can understand.

Table 1 Sales for Carlson Department Store, Sept 09 through Aug 13

Month	2009	2010	2011	2012	2013
January		1.45	2.31	2.31	2.56
February		1.80	1.89	1.99	2.28
March		2.03	2.02	2.42	2.69
April		1.99	2.23	2.45	2.48
May		2.32	2.39	2.57	2.73
June		2.20	2.14	2.42	2.37
July		2.13	2.27	2.40	2.31
August		2.43	2.21	2.50	2.23
September	1.71	1.90	1.89	2.09
October	1.90	2.13	2.29	2.54
November	2.74	2.56	2.83	2.97
December	4.20	4.16	4.04	4.35

Table 2 Department Store Sales for the County, Sept 09 through Dec 13

Month	2009	2010	2011	2012	2013
January		46.8	46.8	43.8	48.0
February		48.0	48.6	45.6	51.6
March		60.0	59.4	57.6	57.6
April		57.6	58.2	53.4	58.2
May		61.8	60.6	56.4	60.0
June		58.2	55.2	52.8	57.0
July		56.4	51.0	54.0	57.6
August		63.0	58.8	60.6	61.8
September	55.8	57.6	49.8	47.4	69.0
October	56.4	53.4	54.6	54.6	75.0
November	71.4	71.4	65.4	67.8	85.2
December	117.6	114.0	102.0	100.2	121.8