Type the next number in this sequence:

1, 3, 6, 10, 15, 21,

1 + 2 = 3

3 + 3 = 6

6 + 4 = 10

10 + 5 = 15

15 + 6 = 21

21 + ? = ?

Add 7 to 21 to find the next number:

21 + 7 = 28

So the next number is 28.