Arithmetic sequence: 6, 9, 12, 15, 18

Check the first differences: if they're constant, add the difference once more for the next term. The correct answer is 21. Traps to avoid: 24 comes from the "doubled step" error; 18 comes from the "repeated last" error; 15 comes from the "subtracted step" error; 30 comes from the "added whole span" error.