The number 1, 3, 6, 10, 15, 21 can be shown as a triangle:
Dots in the rows starting from the bottom row should be like 6, 5, 4, 3, 2, 1.
The top row should always have 1 dot.
So the sequence formed here is in the pattern:
1, 1 + 2, 1 + 2 + 3, 1 + 2 + 3 + 4, and so on.
3