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13P5 = 1287(xPx)

I simplify the above to: 13!/8! = 1287(x!)

The expression on the left simplifies to 154,440.

I divide both sides by 1287 to get: 120=x!

At this point I'm stumped.

Thanks in advance, n

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    If you simply calculate $n!$ for the first few positive integers $n$, you should find your $x$ pretty quickly.2011-07-24

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I am not sure that there is really a better solution than to just calculate the first few factorials at this point and see which one gives you $120$... You should find your answer pretty fast.

It is not possible to invert the "factorial" function in general since it is not injective, i.e. $0!=1!=1$

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    If you did know that you had an exact factorial, and it was much larger than this, Stirling's approximation might come in handy.2011-07-24