Determine what the last number in this series is

4, 23, 20, 12, 14, 9, 20, 19, ?

9

because

In the title of the question, the words begin with DWTLNITSI, corresponding to positions 4, 23, 20, 12, 14, 9, 20, 19, 9 in the alphabet.

• We have a winner! Commented Nov 7, 2014 at 21:33

4, 23, 20, 12, 14, 9, 20, 19, ?
19, -3, -8, 2, -5, 11, -1, ?
-22, -5, -6, -7, -16, -12, ?
17, -1, -1, -9, 4, ?
-18, 0, -8, 13, ?
18, -8, 21, ?
-26, 29, ?
45, ?

We can achieve that by using a difference table. Then starting by the bottom we can work out way up by replacing the first "?" with another 45:

4, 23, 20, 12, 14, 9, 20, 19, 118
19, -3, -8, 2, -5, 11, -1, 99
-22, -5, -6, -7, -16, -12, 100
17, -1, -1, -9, 4, 112
-18, 0, -8, 13, 108
18, -8, 21, 95
-26, 29, 74
45, 45

So the next number in the sequence is 118. Making the full sequence:

4, 23, 20, 12, 14, 9, 20, 19, 118

That is unless I made a mistake calculating

Note: This may not be the answer you're looking for, and I get that, but it doesn't make this answer any less correct; so please do not downvote.

• This is an interesting answer, although definitely not what I'm looking for. Commented Nov 7, 2014 at 21:00
• The problem with this answer is that it can be done for any finite sequence of numbers, and so it can apply to nearly any question. I don't think it's interesting after seeing it the first time. Usually there's a more meaningful answer. Commented Nov 7, 2014 at 21:10
• @warspyking typically for a well made "what's the next number" question, there is a single answer which fits significantly better than any others. Commented Nov 7, 2014 at 21:37
• @war yes, there could be infinite answers. My point is that what distinguishes a good question of this type is that there is a single answer which can be generally agreed to be by far the best fit. If the answer, when revealed, is not obviously correct to most readers I would argue that it is a "read my mind" question. Commented Nov 7, 2014 at 21:45
• The difference between negative 8 and positive 2 is not negative 6. It's Positive 10.
– Dave
Commented Jan 4, 2015 at 22:29