Please explain how to solve this using Excel with cell references

Brothers Willie and Billie each own a gas station for the same national chain. As brothers, they are very competitive. They held a contest to see who did a better job forecasting sales for their station over the course of a year. Willie used a four-week moving average; Billie used a two-week moving average.

a. Create the forecasts for each gas station.

b. Which error calculation would be the best to use in this contest? Why?

c. Who won the contest?

Willie's Petrol Park			Billie's Gasoline Grotto
Week	Sales (1000s of gallons)		Week	Sales (1000s of gallons)
1	17				1	85
2	21				2	84
3	19				3	76
4	23				4	69
5	18				5	54
6	16				6	80
7	20				7	100
8	18				8	54
9	22				9	88
10	20				10	80
11	15				11	60
12	22				12	66