Quick view of 2050 2050 .

What is the smallest value of n n such that 2050 ! 2050! is not divisible by 1 7 n 17^n ?

120 124 126 125 128

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3 solutions

Otto Bretscher
Dec 13, 2015

I cheated and solved this in a second: The answer must be the largest of the given choices, 128 \boxed{128} ;)

I also did the same. @Atul Shivam You should change the ques to lowest possible value of n.

Aditya Chauhan - 5 years, 6 months ago

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Thanks. I've edited the problem accordingly.

Calvin Lin Staff - 5 years, 5 months ago

That ingenious :)

Arulx Z - 5 years, 6 months ago
Akshat Sharda
Dec 13, 2015

Let m m be the highest power of 17 17 which divides 2050 ! 2050! .

m = 2050 17 + 2050 1 7 2 = 120 + 7 = 127 \begin{aligned} m & = \left \lfloor \frac{2050}{17} \right \rfloor + \left \lfloor \frac{2050}{17^2} \right \rfloor \\ & = 120+7 \\ & = 127 \end{aligned}

Therefore, the any value n > m n>m will not divide 2050 ! 2050! .

Answer = 128 128

Oops! I didn't notice your solution.

Arulx Z - 5 years, 6 months ago
Arulx Z
Dec 13, 2015

We just need to count the number of 17s in 2050 ! 2050! . We can simplify the calculation by something like this -

n = 1 k 2050 17 n \sum _{ n=1 }^{ k }{ \left\lfloor \frac { 2050 }{ { 17 }^{ n } } \right\rfloor }

k k must be equal to last n n such that

2050 17 n > 0 \left\lfloor \frac { 2050 }{ { 17 }^{ n } } \right\rfloor >0

After performing the calculations, we get the answer as 127. If 17 127 2050 ! { 17 }^{ 127 }|2050! then every smaller positive integral power less than 127 127 also divides 2050 ! 2050! . Hence only 128 from the list doesn't divide 2050 ! 2050! .

Moderator note:

Simple standard approach.

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