I am attaching the question, I am solving for context.
The sales of the average price of Fiat cars sold in a garage in the Belgian province of Limburg for each month are listed below. The foreman, Mr Verstraten has this data in the period 2010-12.
Mr Verstraten intends to forecast the average car sale prices in each month. Evaluating the data found by Mr Verstraten in 2010-11, find the preliminary estimates for seasonal factors, trend and base. The sales of the average price of Fiat cars sold in a garage in the Belgian province of Limburg for each month are listed below. The foreman, Mr Verstraten has this data in the period 2010-12.
Mr Verstraten intends to forecast the average car sale prices in each month. Evaluating the data found by Mr Verstraten in 2010-11, find the preliminary estimates for seasonal factors, trend and base.
I understand that there is Holt-Winters additive method and Holt-Winters multiplicative method. Now since I am working with monthly data, I know that we are inclined to opt for the additive method "when the seasonal variations are roughly constant through the series ". We would opt for the multiplicative method: when the seasonal variations are changing proportional to the level of the series. I would consider that the question would require me to use the additive method owing to the data that I have collected. And additionally typically in examples of the multiplicative method that I have done in the past there are volatile changes between periods.
For reference I am using this website which elucidates the notion of which method to elect: https://www.otexts.org/fpp/7/5
Here is a time-series plot of the sales for 2010 and 2011:
Does this look like additive seasonality?