The following data report total, monthly U.S. gasoline sales in millions of dollars from January 2017 to December 2018. (To find the data, go to the site https://www.census.gov/retail/index.html#mrts, find "Monthly Retail Trade Report," then select "Time Series/Trend Charts.")

d. Evaluate a seasonally adjusted trend forecasting model. What is the MAD? (Round your answers to 3 decimal places.)

Period	Month-Year	Month	Sales ($ Million)	Trend	Index	Forecast	Absolute Error
1	Jan-2017	1	35,030	36,409.800	0.960
2	Feb-2017	2	33,372	36,828.600	0.906
3	Mar-2017	3	38,022	37,247.400	1.020
4	Apr-2017	4	38,373	37,666.200	1.019
5	May-2017	5	40,139	38,085.000	1.054
6	Jun-2017	6	39,409	38,503.800	1.024
7	Jul-2017	7	39,197	38,922.600	1.007
8	Aug-2017	8	40,617	39,341.400	1.032
9	Sep-2017	9	40,746	39,760.200	1.025
10	Oct-2017	10	41,117	40,179.000	1.023
11	Nov-2017	11	39,586	40,597.800	0.975
12	Dec-2017	12	39,446	41,016.600	0.962
13	Jan-2018	1	38,623	41,435.400	0.932
14	Feb-2018	2	36,777	41,854.200	0.879
15	Mar-2018	3	42,048	42,273.000	0.995
16	Apr-2018	4	42,509	42,691.800	0.996
17	May-2018	5	47,449	43,110.600	1.101
18	Jun-2018	6	46,946	43,529.400	1.078
19	Jul-2018	7	47,391	43,948.200	1.078
20	Aug-2018	8	48,233	44,367.000	1.087
21	Sep-2018	9	44,985	44,785.800	1.004
22	Oct-2018	10	47,742	45,204.600	1.056
23	Nov-2018	11	42,639	45,623.400	0.935
24	Dec-2018	12	39,026	46,042.200	0.848
						MAD=

I am struggling to find the correct formula/process to find the seasonally adjusted forecast and Absolute Error, I did fine with the trend and index on the chart above.

The next question is

e. What are the monthly indexes for months 1-12, I put the numbers I got in the chart above but it shows up as incorrect.

The last question is

f. Forecast sales for January, February, March, and April of 2019.