Unless you specify the length of the sequence, it is infinite, and so no number completes it.
Given no further information, I would say the next number is 18. However, check out the link to the OEIS which gives a number of alternatives.
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The number which best completes the sequence below is:1 1 3 2 4 6 5 25 ?
25
13, 14, 16 - the numbers increase by 1 then 2 each time
The next number would be 18.3, 4, 6, 9, 13To get from 3 to 4, you need to add one. To get from 4 to 6, you need to add two. To get from 6 to 9, you need to add three. And so on and so on.
2 4 3 9 4 16 5 25 6 36 7 49 8 64