3,5,8,13 is followed by 21.
1,1,2,3,5,8,13,21,34,55,... is the Fibonacci series or Fibonacci sequence where the next number in the sequence is the sum of the last two numbers (1+1=2, 1+2=3, 2+3=5, etc.)
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17 cause it is the next prime number
18. 3+1=4, 4+2=6, 6+3=9, 9+4=13, 13+5=18.
The next number is 13... you're adding the previous two numbers together to produce the next in line.
Answer = 21 The next number is the previous 2 numbers added together
11 comes next.