how to compute a seasonal index using ratio to moving average method

Question

givendata on the monthly shipments of air conditioners of a leading manufacturing company from 1983 to 1986;

month	1983	1984	1985	1986
jan	88	168	209	131
feb	130	262	277	175
mar	309	504	530	422
apr	259	459	524	456
may	300	588	632	451
jun	265	648	416	427
jul	306	187	171	361
aug	108	47	68	89
sep	58	40	49	89
oct	32	36	24	53
nov	52	51	39	56
dec	98	113	113	92