The next number is 67.
The simplest polynmial rule is
Un = (8n4 - 97n3 + 412n2 - 713n + 432)/6 for n = 1, 2, 3, ...
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13, 21 - it is the Fibonacci sequence
7, 6.
1+1 = 2 1+2 = 3 2+3 = 5 3+5 = 8 5+8 = 13
5
You add the last two numbers together to get the next number, and 3+5=8, so the answer is 8!