If a, b, c is an arithmetic sequence then a + c = 2b
a = 1/10; b = 3/20, c = 5/30
a + c = 1/10 + 5/30 = 3/30 + 5/30 = (3+5)/30 = 8/30
2b = 2 · 3/20 = 3/10 = 9/30
8/30 ≠ 9/30
If a, b, c is a geometric sequence then ac = b²
ac = 1/10 · 5/30 = 1/2 · 1/30 = 1/60
b² = (3/20)² = 9/400
1/60 ≠ 9/400
Answer: Neither.
