Striking divisibility

Find the smallest positive integer with the property that if you move the first digit to the last then the new number obtained is 1.5 times the original number.

This number is of the form 2 × ( 1 0 16 1 ) n \dfrac{2 \times (10^{16}-1) }{n} . Find n n .


The answer is 17.

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2 solutions

Mark Hennings
Mar 15, 2017

Suppose that the number N N is an m + 1 m+1 digit number, and that the first digit is a a . We can write N = 1 0 m a + b N = 10^ma + b for some integer 0 b < 1 0 m 0 \le b < 10^m . The number obtained by moving the first digit of N N to the end is 10 b + a 10b + a , and so we want 10 b + a = 3 2 ( 1 0 m a + b ) 20 b + 2 a = 3 × 1 0 m a + 3 b 17 b = ( 3 × 1 0 m 2 ) a \begin{aligned} 10b + a & = \tfrac32(10^ma + b) \\ 20b + 2a & = 3\times10^ma + 3b \\ 17b & = (3\times10^m - 2)a \end{aligned} Since a a is a nonzero single digit number, it cannot be divisible by 17 17 , and so we deduce that 17 17 divides 3 × 1 0 m 2 3\times10^m-2 , so that 3 × 1 0 m 2 ( m o d 17 ) 3 \times 10^m \equiv 2 \pmod{17} , or 1 0 m 12 ( m o d 17 ) 10^m \equiv 12 \pmod{17} . The smallest value of m m for which this is true is m = 15 m=15 , and we can have a = 1 b = 1 17 ( 3 × 1 0 15 2 ) a \; = \; 1 \hspace{2cm} b \; = \; \tfrac{1}{17}(3 \times 10^{15} - 2) Choosing a = 1 a=1 with m = 15 m=15 clearly gives us the smallest possible value of N N . Thus N = 1 0 15 + 1 17 ( 3 × 1 0 15 2 ) = 1 17 ( 20 × 1 0 15 2 ) = 2 ( 1 0 16 1 ) 17 N \; = \; 10^{15} + \tfrac{1}{17}(3\times10^{15} - 2) \; = \; \tfrac{1}{17}(20\times10^{15} - 2) \; = \; \frac{2(10^{16} - 1)}{17} which makes the answer 17 \boxed{17} .

Arijit Dey
Mar 4, 2017

[This solution is incomplete.]

Let d be the first digit. Then the number is 10^{k}d +r

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