Explanation

The series is formed by adding 1, 2, 3, 4, 5, ... to the previous term, so the next difference should be 5, making the next term:

• 3 + 1 = 4
• 4 + 2 = 6
• 6 + 3 = 9
• 9 + 4 = 13
• 13 + 5 = 18

Therefore, the completed series is: 3, 4, 6, 9, 13, 18.