Factorial

n ! n! is the factorial of n n , where n ! = 1 2 3 . . . ( n 1 ) n n!=1\cdot 2\cdot 3\cdot ...\cdot \left( n-1 \right) \cdot n . For example 4 ! = 1 2 3 4 = 24 4!=1\cdot 2\cdot 3\cdot 4=24 .

If n ! = 2 15 3 6 5 3 7 2 11 13 n!={ 2 }^{ 15 }\cdot { 3 }^{ 6 }\cdot { 5 }^{ 3 }\cdot { 7 }^{ 2 }\cdot 11\cdot 13 what is the value of n n ?

14 16 13 15

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2 solutions

Prasun Biswas
Jan 19, 2015

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 ! = 16 ! n = 16 n!=16! \implies \boxed{n=16}

Another way is to use reasoning. We see that there are 15 15 2's in the expansion, so using the method of finding trailing zeros, we can also find that if there are 15 15 2's in the expansion, then we have n { 16 , 17 } n\in \{16,17\} since if n < 16 n\lt 16 , there will be less number of 2's and if n > 17 n\gt 17 , there will be more number of 2's. Also, observe that 17 17 is a prime number, so if n = 17 n=17 , there should be a 17 17 in the given prime factorization. But there isn't! So, we have the result n = 16 n=16 assuming the given factorial exists. We can verify the assumption easily.

Hence, the answer is n = 16 \boxed{n=16} _\square

Andrea Virgillito
Apr 29, 2017

2^15 tell us that n must have 15 factors 2, then the unique possible value of n is 16 because there are 8+4+2+1 factors 2. (Because 8 terms of 16! are divisibly by 2 once, but 4 of the latter are divisibly by 2 twice..ecc.)

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