Answer is 123.
Pattern: the difference between the consecutive numbers are multiplied by 3. In this case, the diff is 1 (3 minus 2), 3 (6 minus 3), 9, 27, and 81... and so on...
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(57-42) (24-23) = 15 x 1 = 15. See discussion page.
There are many possible answers. One possible rule is: Un = (3n4 - 18n3 + 69n2 - 54n + 120)/8 for n 1, 2, 3, ...
42: 1, 2, 3, 6, 7, 14, 21, 42.60: 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60.
42 x 2 = 84
(8-2)2+(-4-15)2 = 397 and the square root of this is the distance which is 19.925 rounded to 3 decimal places