The nth term in the sequence is defined by
t(n) = -1 + 4n where n = 1, 2, 3, ...
So t(50) = -1 + 4*50 = -1 + 200 = 199
A number sequence is not a question. So there can be no "answer".
20
14
17
15
17?
15
23
Double it minus the previous number.
The 50th multiple of 15 is 750.
The next number in the sequence 3-15-6-30-21 is -207.
15 is a composite number. 11 is a prime number.