Inspired by Melissa Quail

What is the largest possible integer value of x x such that 17 5 x 175^x divides 1175 ! 1175! ?


Inspiration .

48 6 292 146 193

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1 solution

Vighnesh Raut
Jan 29, 2015

Power of x ,where x is a prime ,in n! is n x + n x 2 + n x 3 + . . . . . + n x a \left\lfloor \frac { n }{ x } \right\rfloor +\left\lfloor \frac { n }{ { x }^{ 2 } } \right\rfloor +\left\lfloor \frac { n }{ { x }^{ 3 } } \right\rfloor +.....+\left\lfloor \frac { n }{ { x }^{ a } } \right\rfloor where the summation ends if the last term is zero.

Note, 175 is not a prime number . So you have to split it into primes and find the powers of individual primes . 175 = 5 2 × 7 175={ 5 }^{ 2 }\times 7 . Power of 5 in 1175! is 292 and of 7 is 193 (we have found the powers by the above formula). But we don't want the power of 5, as for eg if we have three 5's , then there is only one 25.So power of 25 is half the power of 5. We now have power of 25 as 146 and power of 7 as 193 Therefore, for one forty-six 25's we have one forty-six 7's. Using them we can form one forty six 175's .Hence, power of 175 is 146.

Forgot about the extra 5 from factorization! :(

tytan le nguyen - 6 years, 4 months ago

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yeah......Its the common mistake made by many people...

Vighnesh Raut - 6 years, 4 months ago

@Vighnesh Raut I created this for you. If you subscribe to this thread, you will get email notifications about it. Can you improve on the solution? Thanks!

Calvin Lin Staff - 6 years, 4 months ago

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I have edited the solution. Please tell me if there are more flaws.

Vighnesh Raut - 6 years, 4 months ago

Great Solution!

Question: I see that 5 divides 1175 235 times, 25 divides 1175 47 times, and 125 divides 1175 9 times, with remainder 50. 235+47+9 = 291. How do you get 292 powers of 5 in 1175! ?

Thanks!

Rob Matuschek - 5 years, 8 months ago

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What about 625?

Calvin Lin Staff - 5 years, 8 months ago

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