What is the rule for 6 8 11 15 20?

Answer 1

Oh, dude, that's like a sequence question. So, the rule here is that you just add the next number by increasing increments each time. You start with 6, then add 2 to get 8, then add 3 to get 11, then add 4 to get 15, and so on. It's like a number staircase, but with math.

Answer 2

The rule is

Un = 0.5n2 + 0.5n + 5