The numbers 2, $x$, 18 are in geometric progression and the numbers 2, $y$, 18 are in arithmetic progression, with $x$ positive. Then $x+y$ equals:

In a geometric progression the middle term satisfies $x^2=2\times 18=36$, so $x=6$ for the positive choice. In an arithmetic progression the middle term is the average, $y=(2+18)/2=10$. Hence $x+y=16$. (12 uses the geometric mean twice, 20 uses the arithmetic mean twice, and 10 quotes $y$ alone.) The progressions are 2, 6, 18 (ratio 3) and 2, 10, 18 (difference 8).