0, -1.5, -3, -4.5
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The next number in the sequence 3-15-6-30-21 is -207.
I'm guessing your sequence is 1, 3, 6, 10, 15, ... In which case it continues: 21, 28, 36, 45, 55, 66, ... (These are the triangular numbers.)
28
No, according to algebra there is no real answer. But if you take the numbers 35, 45, 50 and 51, it would be 51.125(The number that increases itsself divides by 3 everytime)
3 * 15 = 45 or 15 = 45 / 3 or 3 = 45 / 15