2,7,8,3,12,9 What is the next
number?

4

Explanation:

Taking these numbers as groups of three, we have 2, 7, 8; 3, 12, 9.

In the first group, we can see that we first add 5 and then add 1.

The next group seems to start from the next consecutive number. That is, the first group starts with 2, and the next group starts with 3. The next terms in the second group are found by adding 5 to the second term of the first group (12 = 7+5) and then adding 1 to the third term of the first group (9=8+1); this is because we added 5 and added 1 previously.

Following this pattern, the next group of three would start from the next consecutive number up, 4. The next two terms would be 17 (12+5=17) and 10 (9+1=10).

We have to determine the pattern of numbers followed in the sequence 2, 7, 8, 3, 12, 9.

Do a grouping of the sequence into two groups. Let the first group be 2, 7, 8 and the second group be 3, 12, 9

There is a difference of 1 in the pair 2 and 3, a difference of 5 in the pair 7 and 12 and a difference of 1 in the pair 8 and 9

This means there would be a difference of 5 in the next pair.

So, adding 5 to the first number to the third pair.