1/10, 3/20, 5/30, 7/40 arithmetic sequence geometric sequence neither

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

Neither.