Please help to quote relevant formulas and show the steps of the working. thanks

Question 4 The following data in the sales in ice-cream was collected in the past 2 years in each quarter. Time period Sales (in HK$000) 15t Q, 2020 80 2\" Q, 2020 35 3"'11 Q, 2020 82 4'h C1. 2020 \"FE 15t Q, 2021 78 2"Id Q, 2021 88 3"\"I Q, 2021 85 ail"1 Q, 2021 80 Total 654 a. Find out the seasonal index for 1st Q to 4'\" 0 using the above data. {15 marks) b. Find out the deseasonalized value for 1Sl Q to 4'l1 Q of 2021. (0 marks} c. According to your answer above, which quarter has the highest sales after taking away the seasonal effect? (2 marks) Please quote relevant formula and show the steps when working out your answers