I need help calculating ratios. These are the answers I currently have, but I am not sure on many of them. I don't know how to do average payment period at all. I also am unsure of which number to be using for net sales, I have been using either operating revenue or total passenger revenue as sales.

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Dec. 31, 2017 Dec. 31, 2016 Dec 31, 2015 Dec 31, 2014 $ 29,105 5,714 34,819 729 5,696 41,244 $ 28,105 5,672 33,777 668 5,194 39,639 $ 28,898 5,884 34,782 813 5,109 40,704 $ 28,688 6,266 34,954 934 4,474 40,362 Passenger: Mainline Regional carriers Total passenger revenue Cargo Other Total operating revenue Operating Expense: Salaries and related costs Aircraft fuel and related taxes Regional carriers expense Depreciation and amortization Contracted services Aircraft maintenance materials and outside repairs Passenger commissions and other selling expenses Landing fees and other rents Passenger service Profit sharing Aircraft rent Other Total operating expense Operating Income Non-Operating Expense: Interest expense, net Miscellaneous, net Total non-operating expense, net Income Before Income Taxes Income Tax Provision Net Income 10,436 5,733 4,503 2,235 2,184 1,992 1,787 1,528 1,067 1,065 351 2,249 35,130 6,114 10,034 5,133 4,311 1,902 1,991 1,823 1,710 1,490 907 1,115 285 1,986 32,687 6,952 8,776 6,544 4,241 1,835 1,848 1,848 1,672 1,493 872 1,490 250 2,033 32,902 7,802 8,120 11,668 5,237 1,771 1,749 1,828 1,700 1,442 810 1,085 233 2,513 38,156 2,206 (396) (17) (413) 5,701 12,124) $ 3,577 (388) 72 (316) 6,636 (2,263) $ 4,373 (481) (164) (645) 7,157 (2,631) $ 4,526 (650) (484) (1,134) 1,072 (413) $ 659 Dec 31, 2017 Dec 31, 2016 Dec. 31, 2015 Dec. 31, 2014 $ 1,814 825 2,377 $ 2,762 487 2,064 $ 1,972 1,465 2,020 $ 2,088 1,217 2,297 916 413 519 372 119 379 318 925 534 318 1,987 796 1,499 7.844 1247 7.451 1,078 701 9,158 9,056 26,563 24,375 23,039 21,929 9,794 4.847 Consolidated Balance Sheets - USD ($) $ in Millions Current Assets: Cash and cash equivalents Short-term investments Accounts receivable, net of an allowance for uncollectible accounts of $12 and $15 at December 31, 2017 and 2016, respectively Hedge margin receivable Fuel inventory Expendable parts and supplies inventories, net of an allowance for obsolescence of $113 and $110 at December 31, 2017 and 2016, respectively Hedge derivatives asset Prepaid expenses and other Total current assets Property and Equipment, Net: Property and equipment, net of accumulated depreciation and amortization of $14,097 and $12,456 at December 31, 2017 and 2016, respectively Other Assets: Goodwill dentifiable intangibles, net of accumulated amortization of $845 and $828 at December 31, 2017 and 2016, respectively Deferred income taxes, net ther noncurrent assets Total other assets Total assets Current Liabilities: Current maturities of long-term debt and capital leases Bir traffic liability Accounts payable Accrued salaries and related benefits Frequent flyer deferred revenue Fuel oard obligation Other accrued liabilities Total current liabilities Voncurrent Liabilities: ong-term debt and capital leases Pension, postretirement and related benefits Frequent flyer deferred revenue Other noncurrent liabilities 9,794 4,844 9,794 4,861 9,794 4,603 935 3,309 18,885 53,292 3,064 1,733 19,435 51,261 4,956 1,428 21,039 53,134 7,595 926 22,918 54,005 2,242 4,888 3,674 3,022 1,822 1,067 1,858 18,573 1,131 4,626 2,572 2,924 1,648 431 1,907 15,239 1563 4,503 2,743 3,195 2,581 1635 1,306 17,526 1,184 4.296 2,622 2,266 2,772 1580 2,127 16,847 6,592 9,810 2,296 2,111 6,201 13,378 2,278 1,878 6,766 13,855 2.246 1.891 8,477 15,138 2,602 2,128 2,242 4,888 3,674 3,022 1,822 1,067 1,858 18,573 1,131 4,626 2.572 2,924 1,648 431 1,907 15,239 1563 4,503 2,743 3,195 2,581 1635 1,306 17,526 1,184 4,296 2,622 2,266 2,772 1,580 2,127 16,847 Current Liabilities: Current maturities of long-term debt and capital leases Air traffic liability Accounts payable Accrued salaries and related benefits Frequent flyer deferred revenue Fuel oard obligation Other accrued liabilities Total current liabilities Noncurrent Liabilities: Long-term debt and capital leases Pension, postretirement and related benefits Frequent flyer deferred revenue Other noncurrent liabilities Total noncurrent liabilities Commitments and Contingencies Stockholders' Equity: Common stock at $0.0001 par value: 1,500,000,000 shares authorized, 714,674,160 and 744,886,938 shares issued at December 31, 2017 and 2016, respectively Additional paid-in capital Retained earnings Accumulated other comprehensive loss Treasury stock, at cost, 7,476,181 and 14,149,229 shares at December 31, 2017 and 2016, respectively Total stockholders' equity Total liabilities and stockholders' equity 6,592 9,810 2,296 2,111 20,809 6,201 13,378 2,278 1,878 23,735 6,766 13,855 2,246 1,891 24,758 8,477 15,138 2,602 2,128 $ 28,345 0 0 0 $0 12,053 9,636 (7.621) (158) 12,294 7,903 (7.636) (274) 12,936 5,562 (7.275) (373) 12,981 3,456 (7,311) (313) 13,910 $ 53,292 12,287 $ 51,261 10,850 $53,134 8,813 $ 54,005 Consolidated Statements of Cash Flows - USD ($) $ in Millions 12 Months Ended Dec 31, 2017 Dec 31, 2016 Dec 31, 2015 Dec. 31, 2014 $3,577 $4,373 $4,526 $ 659 2,235 (7) 2,071 (3,302) (1) 1,902 (342) 2,223 (717) (160) 1.835 (1,366) 2,581 (1,013) (35) 1,771 2,186 414 (723) 758 268 106 Cash Flows From Operating Activities: Net income Adjustments to reconcile net income to net cash provided by Depreciation and amortization Hedge derivative contracts Deferred income taxes Pension postretirement and postemployment payments greater than expense Equity investment earnings Changes in certain assets and liabilities: Receivables Fuel inventory Hedge margin Prepaid expenses and other current assets Air traffic liability Frequent flyer deferred revenue Profit sharing Accounts payable and accrued liabilities Other, net Net cash provided by operating activities Property and equipment additions: Flight equipment, including advance payments Ground property and equipment, including technology Purchase of equity investments Purchase of short-term investments Redemption of short-term investments Other, net Net cash used in investing activities Cash Flows From Financing Activities: Payments on long-term debt and capital lease obligations Repurchase of common stook (328) (397) (5) (57) 262 192 (51) 992 (33) 5,148 (147) (140) 81 (26) 123 45 (383) 285 88 7,205 (56) 155 806 (102) 207 (301) 734 (201) 157 7,927 (302) 62 172 (922) 58 174 (238) 264 (36) 276 4,947 (2,704) (1,187) (1,245) (925) 584 111 (5,366) (2,617) (774) 0 (1,707) 2,686 257 (2,155) (2,223) (722) (500) (998) 739 (251) (3,955) (1,662) (587) 0 (1,795) 1,533 (1.258) (1,677) (1,709) (2,601) (2,558) (2,200) OO 1,533 0 0 48 (2,463) Cash Flows From Financing Activities: Payments on long-term debt and capital lease obligations Repurchase of common stock Cash dividends Fuel card obligation Payments on hedge derivative contracts Proceeds from hedge derivative contracts Proceeds from long-term obligations Other, net Net cash used in financing activities Net (Decrease) Increase in Cash and Cash Equivalents Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period Supplemental Disclosure of Cash Paid for Interest Non-Cash Transactions: Treasury stock contributed to our qualified defined benefit pension plans Flight and ground equipment acquired under capital leases (1,258) (1,677) (731) 636 (244) 20 2,454 70 (730) (948) 2,762 1,814 390 (1,709) (2,601) (509) 211 (451) 291 450 58 (4,260) 790 1.972 2,762 385 (2,558) (2,200) (359) (340) (71) 429 1,038 (27) (4,088) (116) 2,088 1972 452 (2.928) (1,100) (251) (41) 0 0 1,020 60 (3,240) (756) 2,844 350 261 350 86 0 111 Dec. 31, 2017 Dec. 31, 2016 Dec. 31, 2015 Dec. 31, 2014 Delta Airlines LIQUIDITY RATIOS Current ratio (times) Quick ratio (times) Average payment period (days) 0.42 0.37 0.49 0.45 0.52 0.50 0.54 0.51 ASSET MANAGEMENT RATIOS Total asset turnover (times) Fixed asset turnover (times) Average collection period (days) Inventory turnover (times) 0.79 2.15 19.65 7.99 0.76 1.96 18.80 11.43 0.76 1.85 19.36 14.34 0.75 1.76 20.77 21.85 FINANCIAL LEVERAGE RATIOS Total debt to total assets Equity multiplier (times) Interest coverage (times) 73.90% 3.83 14.40 76.03% 4.17 17.10 79.58% 4.90 14.88 83.68% 6.13 1.65 PROFITABILITY RATIOS Operating profit margin Net profit margin Operating return on assets Return on total assets Return on equity 17.56% 10.27% 11.70% 6.71% 25.72% 20.58% 12.95% 13.32% 8.53% 35.59% 22.43% 13.01% 14.56% 8.52% 41.71% 6.31% 1.89% 8.17% 1.22% 7.48% Dec. 31, 2017 Dec. 31, 2016 Dec 31, 2015 Dec 31, 2014 $ 29,105 5,714 34,819 729 5,696 41,244 $ 28,105 5,672 33,777 668 5,194 39,639 $ 28,898 5,884 34,782 813 5,109 40,704 $ 28,688 6,266 34,954 934 4,474 40,362 Passenger: Mainline Regional carriers Total passenger revenue Cargo Other Total operating revenue Operating Expense: Salaries and related costs Aircraft fuel and related taxes Regional carriers expense Depreciation and amortization Contracted services Aircraft maintenance materials and outside repairs Passenger commissions and other selling expenses Landing fees and other rents Passenger service Profit sharing Aircraft rent Other Total operating expense Operating Income Non-Operating Expense: Interest expense, net Miscellaneous, net Total non-operating expense, net Income Before Income Taxes Income Tax Provision Net Income 10,436 5,733 4,503 2,235 2,184 1,992 1,787 1,528 1,067 1,065 351 2,249 35,130 6,114 10,034 5,133 4,311 1,902 1,991 1,823 1,710 1,490 907 1,115 285 1,986 32,687 6,952 8,776 6,544 4,241 1,835 1,848 1,848 1,672 1,493 872 1,490 250 2,033 32,902 7,802 8,120 11,668 5,237 1,771 1,749 1,828 1,700 1,442 810 1,085 233 2,513 38,156 2,206 (396) (17) (413) 5,701 12,124) $ 3,577 (388) 72 (316) 6,636 (2,263) $ 4,373 (481) (164) (645) 7,157 (2,631) $ 4,526 (650) (484) (1,134) 1,072 (413) $ 659 Dec 31, 2017 Dec 31, 2016 Dec. 31, 2015 Dec. 31, 2014 $ 1,814 825 2,377 $ 2,762 487 2,064 $ 1,972 1,465 2,020 $ 2,088 1,217 2,297 916 413 519 372 119 379 318 925 534 318 1,987 796 1,499 7.844 1247 7.451 1,078 701 9,158 9,056 26,563 24,375 23,039 21,929 9,794 4.847 Consolidated Balance Sheets - USD ($) $ in Millions Current Assets: Cash and cash equivalents Short-term investments Accounts receivable, net of an allowance for uncollectible accounts of $12 and $15 at December 31, 2017 and 2016, respectively Hedge margin receivable Fuel inventory Expendable parts and supplies inventories, net of an allowance for obsolescence of $113 and $110 at December 31, 2017 and 2016, respectively Hedge derivatives asset Prepaid expenses and other Total current assets Property and Equipment, Net: Property and equipment, net of accumulated depreciation and amortization of $14,097 and $12,456 at December 31, 2017 and 2016, respectively Other Assets: Goodwill dentifiable intangibles, net of accumulated amortization of $845 and $828 at December 31, 2017 and 2016, respectively Deferred income taxes, net ther noncurrent assets Total other assets Total assets Current Liabilities: Current maturities of long-term debt and capital leases Bir traffic liability Accounts payable Accrued salaries and related benefits Frequent flyer deferred revenue Fuel oard obligation Other accrued liabilities Total current liabilities Voncurrent Liabilities: ong-term debt and capital leases Pension, postretirement and related benefits Frequent flyer deferred revenue Other noncurrent liabilities 9,794 4,844 9,794 4,861 9,794 4,603 935 3,309 18,885 53,292 3,064 1,733 19,435 51,261 4,956 1,428 21,039 53,134 7,595 926 22,918 54,005 2,242 4,888 3,674 3,022 1,822 1,067 1,858 18,573 1,131 4,626 2,572 2,924 1,648 431 1,907 15,239 1563 4,503 2,743 3,195 2,581 1635 1,306 17,526 1,184 4.296 2,622 2,266 2,772 1580 2,127 16,847 6,592 9,810 2,296 2,111 6,201 13,378 2,278 1,878 6,766 13,855 2.246 1.891 8,477 15,138 2,602 2,128 2,242 4,888 3,674 3,022 1,822 1,067 1,858 18,573 1,131 4,626 2.572 2,924 1,648 431 1,907 15,239 1563 4,503 2,743 3,195 2,581 1635 1,306 17,526 1,184 4,296 2,622 2,266 2,772 1,580 2,127 16,847 Current Liabilities: Current maturities of long-term debt and capital leases Air traffic liability Accounts payable Accrued salaries and related benefits Frequent flyer deferred revenue Fuel oard obligation Other accrued liabilities Total current liabilities Noncurrent Liabilities: Long-term debt and capital leases Pension, postretirement and related benefits Frequent flyer deferred revenue Other noncurrent liabilities Total noncurrent liabilities Commitments and Contingencies Stockholders' Equity: Common stock at $0.0001 par value: 1,500,000,000 shares authorized, 714,674,160 and 744,886,938 shares issued at December 31, 2017 and 2016, respectively Additional paid-in capital Retained earnings Accumulated other comprehensive loss Treasury stock, at cost, 7,476,181 and 14,149,229 shares at December 31, 2017 and 2016, respectively Total stockholders' equity Total liabilities and stockholders' equity 6,592 9,810 2,296 2,111 20,809 6,201 13,378 2,278 1,878 23,735 6,766 13,855 2,246 1,891 24,758 8,477 15,138 2,602 2,128 $ 28,345 0 0 0 $0 12,053 9,636 (7.621) (158) 12,294 7,903 (7.636) (274) 12,936 5,562 (7.275) (373) 12,981 3,456 (7,311) (313) 13,910 $ 53,292 12,287 $ 51,261 10,850 $53,134 8,813 $ 54,005 Consolidated Statements of Cash Flows - USD ($) $ in Millions 12 Months Ended Dec 31, 2017 Dec 31, 2016 Dec 31, 2015 Dec. 31, 2014 $3,577 $4,373 $4,526 $ 659 2,235 (7) 2,071 (3,302) (1) 1,902 (342) 2,223 (717) (160) 1.835 (1,366) 2,581 (1,013) (35) 1,771 2,186 414 (723) 758 268 106 Cash Flows From Operating Activities: Net income Adjustments to reconcile net income to net cash provided by Depreciation and amortization Hedge derivative contracts Deferred income taxes Pension postretirement and postemployment payments greater than expense Equity investment earnings Changes in certain assets and liabilities: Receivables Fuel inventory Hedge margin Prepaid expenses and other current assets Air traffic liability Frequent flyer deferred revenue Profit sharing Accounts payable and accrued liabilities Other, net Net cash provided by operating activities Property and equipment additions: Flight equipment, including advance payments Ground property and equipment, including technology Purchase of equity investments Purchase of short-term investments Redemption of short-term investments Other, net Net cash used in investing activities Cash Flows From Financing Activities: Payments on long-term debt and capital lease obligations Repurchase of common stook (328) (397) (5) (57) 262 192 (51) 992 (33) 5,148 (147) (140) 81 (26) 123 45 (383) 285 88 7,205 (56) 155 806 (102) 207 (301) 734 (201) 157 7,927 (302) 62 172 (922) 58 174 (238) 264 (36) 276 4,947 (2,704) (1,187) (1,245) (925) 584 111 (5,366) (2,617) (774) 0 (1,707) 2,686 257 (2,155) (2,223) (722) (500) (998) 739 (251) (3,955) (1,662) (587) 0 (1,795) 1,533 (1.258) (1,677) (1,709) (2,601) (2,558) (2,200) OO 1,533 0 0 48 (2,463) Cash Flows From Financing Activities: Payments on long-term debt and capital lease obligations Repurchase of common stock Cash dividends Fuel card obligation Payments on hedge derivative contracts Proceeds from hedge derivative contracts Proceeds from long-term obligations Other, net Net cash used in financing activities Net (Decrease) Increase in Cash and Cash Equivalents Cash and cash equivalents at beginning of period Cash and cash equivalents at end of period Supplemental Disclosure of Cash Paid for Interest Non-Cash Transactions: Treasury stock contributed to our qualified defined benefit pension plans Flight and ground equipment acquired under capital leases (1,258) (1,677) (731) 636 (244) 20 2,454 70 (730) (948) 2,762 1,814 390 (1,709) (2,601) (509) 211 (451) 291 450 58 (4,260) 790 1.972 2,762 385 (2,558) (2,200) (359) (340) (71) 429 1,038 (27) (4,088) (116) 2,088 1972 452 (2.928) (1,100) (251) (41) 0 0 1,020 60 (3,240) (756) 2,844 350 261 350 86 0 111 Dec. 31, 2017 Dec. 31, 2016 Dec. 31, 2015 Dec. 31, 2014 Delta Airlines LIQUIDITY RATIOS Current ratio (times) Quick ratio (times) Average payment period (days) 0.42 0.37 0.49 0.45 0.52 0.50 0.54 0.51 ASSET MANAGEMENT RATIOS Total asset turnover (times) Fixed asset turnover (times) Average collection period (days) Inventory turnover (times) 0.79 2.15 19.65 7.99 0.76 1.96 18.80 11.43 0.76 1.85 19.36 14.34 0.75 1.76 20.77 21.85 FINANCIAL LEVERAGE RATIOS Total debt to total assets Equity multiplier (times) Interest coverage (times) 73.90% 3.83 14.40 76.03% 4.17 17.10 79.58% 4.90 14.88 83.68% 6.13 1.65 PROFITABILITY RATIOS Operating profit margin Net profit margin Operating return on assets Return on total assets Return on equity 17.56% 10.27% 11.70% 6.71% 25.72% 20.58% 12.95% 13.32% 8.53% 35.59% 22.43% 13.01% 14.56% 8.52% 41.71% 6.31% 1.89% 8.17% 1.22% 7.48%