39 The Fowler Martial Arts Academy trains young boys and girls in self-defense. Joan Fowler, the owner of the academy, notes that monthly revenue is higher when school is in session but quite low when school is out (because many children are away on vacation or at summer camp). She has researched revenues for the past four years and obtained the information shown in the Table for Problem 39.

Decompose Joan’s data by using a multiplicative model. Use the model to forecast revenues for year 5. Comment on the validity of the model.

MONTH YEAR 1 YEAR 2 YEAR 3 YEAR 4 January $59,042 $57,495 $56,583 $55,658 February $62,659 $62,622 $66,438 $67,164 March $22,879 $24,273 $27,766 $27,795 April $29,946 $30,492 $31,600 $30,667 May $26,839 $28,237 $29,589 $31,962 June $19,134 $17,893 $20,115 $21,096 July $20,051 $21,126 $19,324 $22,778 August $19,625 $22,876 $23,486 $23,144 September $19,925 $22,641 $24,719 $26,601 October $58,435 $60,796 $60,666 $61,385 November $87,705 $87,815 $86,693 $88,581 December $77,430 $78,711 $80,056 $81,048