is the factorial of , where . For example .
If what is the value of ?
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Well, there are two ways of solving this. One way is to rearrange the given value in the form of a factorial and keep count of the prime factors to get, n ! = 1 6 ! ⟹ n = 1 6
Another way is to use reasoning. We see that there are 1 5 2's in the expansion, so using the method of finding trailing zeros, we can also find that if there are 1 5 2's in the expansion, then we have n ∈ { 1 6 , 1 7 } since if n < 1 6 , there will be less number of 2's and if n > 1 7 , there will be more number of 2's. Also, observe that 1 7 is a prime number, so if n = 1 7 , there should be a 1 7 in the given prime factorization. But there isn't! So, we have the result n = 1 6 assuming the given factorial exists. We can verify the assumption easily.
Hence, the answer is n = 1 6 □