What's the missing number?
Admit it, you think it's 5, don't you? Occam's Razor and all that.
Stop thinking so linearly. I'm all for keeping things simple, but you need to let your hair down once in a while.
The answer I'm looking for is 17.
Why? I'd have thought that much was obvious: 1, 2, 3, 4 and 17 are the five solutions to the polynomial equation:
x5 - 27x4 + 205x3 - 645x2 +874x - 408 = 0
Like all good puzzles, it's obvious when you know the answer.
Good job that didn't come up in an IQ test, eh?