The following data are quarterly sales of natural gas in Saskatchewan by SaskEnergy (in peta joules 1 billion cubic feet) from Q1 of2005 to Q3 of2009.LL

a. Compute the seasonal relative for each quarter using the centred moving average method.
b. Deseasonalize the data, fit an appropriate model to the deseasonalized data, extend the model four quarters, and reseasonaiize these in order to forecast the sales of natural gas by SaskEnergy from Q4 2009 to Q3 2010.