Question

3, -6, 12, 4, 20, ? What is the missing term u

Answer 1

13

Explanation:

Odd-numbered terms of the sequence appear to have first differences of 9, 8, 7, ...

Even-numbered terms of the sequence appear to have first differences of 10, 9, 8, ...

Thus the 6th term of the sequence will be 9 more than the 4th term of the sequence. It will be 13.

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In the absence of any clue as to the nature of the sequence, you can choose the next term(s) to be anything you like. A rule can always be made that will fit the term values you choose.