( 201 9 11 ) ! (2019_{11}) !

How many trailing zeroes does the number ( 201 9 11 ) ! (2019_{11}) ! have?


The answer is 267.

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1 solution

David Vreken
Dec 30, 2018

In base 10 10 , 201 9 11 2019_{11} is 2 1 1 3 + 0 1 1 2 + 1 1 1 1 + 9 1 1 0 = 268 2 10 2 \cdot 11^3 + 0 \cdot 11^2 + 1 \cdot 11^1 + 9 \cdot 11^0 = 2682_{10} , so ( 201 9 11 ) ! = ( 268 2 10 ) ! (2019_{11})! = (2682_{10})! .

The number of trailing zeros a number has in base 11 11 is the same as the number of factors of 11 11 it has. Since 2682 ! = 2682 2681 2680 3 2 1 2682! = 2682 \cdot 2681 \cdot 2680 \dots 3 \cdot 2 \cdot 1 , int ( 2682 11 ) = 243 \text{int}(\frac{2682}{11}) = 243 of these numbers are multiples of 11 11 , int ( 2682 1 1 2 ) = 22 \text{int}(\frac{2682}{11^2}) = 22 of these numbers are multiples of 1 1 2 11^2 , and int ( 2682 1 1 3 ) = 2 \text{int}(\frac{2682}{11^3}) = 2 of these numbers are multiples of 1 1 3 11^3 , so 2682 ! 2682! has at most 243 + 22 + 2 = 267 243 + 22 + 2 = 267 factors of 11 11 , which means there are 267 \boxed{267} trailing zeros in ( 201 9 11 ) ! (2019_{11})!

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