1 3 6 10 15 | What is the next number in the sequence 1 3 6 10 15 ?

Find the next number that fits the pattern:

1, 3, 6, 10, 15, ?

21

Explanation:

3 = 1 + 2
6 = 3 + 3
10 = 6 + 4
15 = 10 + 5
21 = 15 + 6

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