I have answers with the steps included above but I need explanations how/why

Example 2. Given the following information, compute the gross profit for the year. Net Accounts receivable beginning $55,000 Net Accounts receivable End $59,000 Days Accounts receivable outstanding 19.4 Inventory beginning $16,100 Inventory end $16,800 Days Inventory held 12.2 rec turn 18.81443299 $1,072,422.68 Net credit sales inv turn COGS 29.91803279 $492,151.64 $580,271.04 Gross Profit