1 8 17 30 __ 76 Differences 7 9 13 __ __ Diff of Diff 2 4 Diff of diff of diff 2 Presuming the difference of the differences of the differences is constant, and then working back UP the table: 1 8 17 30 49 76 Differences 7 9 13 19 27 Diff of Diff 2 4 6 8 Diff of diff of diff 2 2 2