Explanation:

Alright, let’s break down this number series: 1, 7, 3, 9, 6, 12, 10, 16, 15, ?

- If you look closely, the series is in pairs:

- 1, 7

- 3, 9

- 6, 12

- 10, 16

- 15, ?

- The first number in each pair goes like this: 1, 3, 6, 10, 15.

- That’s a common sequence called “triangular numbers”—each is made by adding the next natural number (1, +2, +3, +4, +5).

- 1 ? 3 (1+2)

- 3 ? 6 (3+3)

- 6 ? 10 (6+4)

- 10 ? 15 (10+5)

- So, next would be 15+6 = 21.

- The second number in each pair follows: 7, 9, 12, 16.

- Pattern: +2, +3, +4

- 7 ? 9 (+2)

- 9 ? 12 (+3)

- 12 ? 16 (+4)

- Next would be 16 + 5 = 21, but the question is about the first number, so not needed!

- The missing term is 21.

Let’s check your answer.

Here’s that green tick for you: Option 4 – 21 is the right call.