Moving Averages

Good day, guys may those who are familiar with moving averages kindly assist me in tackling this question.I need a step by step guide on calculation of moving averages.

Set out below are the sales per quarter (in 000’s of units) of a company over the last 3 years.

Quarter
1 2 3 4
2000 80 87 82 90
2001 90 95 93 102
2002 105 112 103 116

Required

a) Identify the trend and calculate the average seasonal variation.
b) Calculate the average seasonal variation using the multiplicative model.

I look forward to your assistance.