1, 2, 3, 4, __

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?


5 thoughts on “1, 2, 3, 4, __

  1. There is something askew with this comment thread - it seems to relate to two entries (1-2-3-4 and Celebrity Iraq watch).

    I was simply going to ask whether you devised the cited equation, and if so why. Since I'm apparently commenting on Iraq-watch, however, I will agree that it is slightly bizarre to expect us to care any more about what a "celebrity" thinks than anyone else, but then this is the basis of many charity appeals and so forth, so it's hardly unique.

  2. I've fixed the comments bug (caused by my assigning the same 'unique' reference to two different posts - I maintain this crap by hand, you see).

    As to why I devised the equation, it was merely to prove that the correct answer is 17.

  3. No criticism intended. Just thought I'd mention it.

    Sadly, my maths is not longer up to solving such things, far less devising them.

  4. Have to say, I don't think I'd be able to solve it either. But the devising is pretty simple... You just have to multiply out:

    (x -1)(x - 2)(x - 3)(x - 4)(x - 17) = 0

  5. Ah yes - that would do it.

    To quote Charlie Drake "Hadn't thought 'a that. He said"

    Another one that's easy when you know how.

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