What is the pattern of 1,3,6,10,15 and the next number in the pattern?

The next number is 21

3 = 1 + 2

6 = 3 + 3

10 = 6 + 4

15 = 10 + 5

21 = 15 + 6

The pattern here is that for each number, they are adding 1 more than they added to the previous number. We can see this because they add 1 and 2 to make 3, 3 and 3 to make 6, 6 and 4 to make 10, and 10 and 5 to make 15.

With this logic, we know the next number in the pattern is 21 because 15+6 is 21.