This is Fibonacci's sequence, where the previous two numbers add up to give you the new number, so it would go:
1, 1, 2, 3, 5, 13, 18, 31, 49 etc
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24
21. Add the last 2 numbers to get the next.
From the pattern (n + 4, n + 2, n + ?) I would say the next following number is 20 (n + 1).
The next number in the sequence 5-7-13-31 is 69.
The next number will be 13+21 = 34