What are the rules of 3, 6, 10, 15 and 21?
Starting from the second term, the difference between each term and the previous term is a natural sequence starting with 3
For example, 6-3 = 3;
10-6=4;
15-10=5...
The general formula is an = 2n + 1 + n (n-1) / 2 (n = 1,2,3,4,5)