what was the average upward revisions of cash flow of previous quantity estimates during the three year period , in millions?
supplementary information on oil and gas producing activities ( unaudited ) c o n t i n u e d summary of changes in standardized measure of discounted future net cash flows relating to proved oil and gas reserves ( in millions ) 2006 2005 2004 sales and transfers of oil and gas produced , net of production , transportation and administrative costs $ ( 5312 ) $ ( 3754 ) $ ( 2689 ) net changes in prices and production , transportation and administrative costs related to future production ( 1342 ) 6648 771 .

| <i>(In millions)</i> | 2006 | 2005 | 2004 |
| :--- | :--- | :--- | :--- |
| Sales and transfers of oil and gas produced, net of production, transportation and administrative costs | $(5,312) | $(3,754) | $(2,689) |
| Net changes in prices and production, transportation and administrative costs related to future production | (1,342) | 6,648 | 771 |
| Extensions, discoveries and improved recovery, less related costs | 1,290 | 700 | 1,349 |
| Development costs incurred during the period | 1,251 | 1,030 | 609 |
| Changes in estimated future development costs | (527) | (552) | (628) |
| Revisions of previous quantity estimates | 1,319 | 820 | 948 |
| Net changes in purchases and sales of minerals in place | 30 | 4,557 | 33 |
| Accretion of discount | 1,882 | 1,124 | 757 |
| Net change in income taxes | (660) | (6,694) | (627) |
| Timing and other | (14) | 307 | 97 |
| Net change for the year | (2,083) | 4,186 | 620 |
| Beginning of year | 10,601 | 6,415 | 5,795 |
| End of year | $8,518 | $10,601 | $6,415 |
| Net change for the year from discontinued operations | $(216) | $162 | $(152) |

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table_row = [1319, 820, 948] # row labeled revisions of previous quantity estimates
a = sum(table_row)/len(table_row)