The next number would be 18.
3, 4, 6, 9, 13
To 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.
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The number which best completes the sequence below is:1 1 3 2 4 6 5 25 ?
The sequence appears to alternate between adding and subtracting numbers. The pattern seems to be +11, -7, +5, -2, +6. Following this pattern, the next number in the sequence would be +3 from the previous number, which would be 21.
25
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.
The sequence appears to be alternating between addition and subtraction operations. Starting with 7, we subtract 6 to get 1, then add 4 to get 5, subtract 8 to get -3, and then add 10 to get 7. Following this pattern, the next operation would be subtraction, so we subtract 12 from 7 to get -5. Therefore, the number that best completes the sequence is -5.