Explanation:

Let’s break this sequence down:

- The numbers are: 15  81  1218  99  1517  120  ?

- Now look closer at each pair:

- 15 (1 and 5)

- 81 (8 and 1)

- 1218 (12 and 18)

- 99 (9 and 9)

- 1517 (15 and 17)

- 120 (12 and 0)

- The pattern: Each is made from two numbers run together, and each consecutive pair has a first number and a second number with a difference of 2 or 7:

- 1,5 (difference 4)

- 8,1 (difference 7)

- 12,18 (difference 6)

- 9,9 (difference 0)

- 15,17 (difference 2)

- 12,0 (difference 12)

- Wait—so it shifts pattern. Let’s see another way: Consecutive numbers. Check the first digit in each number: 1,8,12,9,15,12. These don’t repeat, but the second digits are 5,1,18,9,17,0—still don’t see a basic arithmetical pattern.

- But looking closer, the likely rule could be: each number is the sum of the previous first and second digit (ex: 1+5=6, 8+1=9, etc.)—but that doesn’t fit cleanly.

- Let's try pairing: The first numbers in the pair: 1, 8, 12, 9, 15, 12. They do not form an arithmetic or geometric progression.

- Now, check the difference between numbers:

- 81 - 15 = 66

- 1218 - 81 = 1137

- 99 - 1218 = -1119

- 1517 - 99 = 1418

- 120 - 1517 = -1397

That doesn’t fit either.

Maybe the direct pattern is simpler: check the next digit after 120, which is (12,0), what makes sense? The options are:

- 13

- 11

- 9

- 16

Looking at the sequence of the first numbers: 1, 8, 12, 9, 15, 12. There’s a jump by +7, +4, -3, +6, -3.

It looks like the first, third, and fifth numbers: 1, 12, 15; 1+11=12, 12+3=15; and the second, fourth, sixth: 8, 9, 12; 8+1=9, 9+3=12.

Maybe the sequence alternates between adding 1, then 3.

But it's honestly a pretty odd sequence and doesn’t map perfectly to any regular arithmetic pattern. The closest is to keep the “increase by 1, then by 3” style, so after 12 (last first number), you’d add 1 (12+1=13).

So the answer should be 13.  

Option:1, 13 is correct.

To sum it up:

- There’s no obvious arithmetic logic; it most closely follows a pattern of adding 1, then 3, so after 12, likely is 13.

- Option:1, 13 is the only one that fits most cleanly with that sawtooth pattern.