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 . Find .
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Suppose that the number N is an m + 1 digit number, and that the first digit is a . We can write N = 1 0 m a + b for some integer 0 ≤ b < 1 0 m . The number obtained by moving the first digit of N to the end is 1 0 b + a , and so we want 1 0 b + a 2 0 b + 2 a 1 7 b = 2 3 ( 1 0 m a + b ) = 3 × 1 0 m a + 3 b = ( 3 × 1 0 m − 2 ) a Since a is a nonzero single digit number, it cannot be divisible by 1 7 , and so we deduce that 1 7 divides 3 × 1 0 m − 2 , so that 3 × 1 0 m ≡ 2 ( m o d 1 7 ) , or 1 0 m ≡ 1 2 ( m o d 1 7 ) . The smallest value of m for which this is true is m = 1 5 , and we can have a = 1 b = 1 7 1 ( 3 × 1 0 1 5 − 2 ) Choosing a = 1 with m = 1 5 clearly gives us the smallest possible value of N . Thus N = 1 0 1 5 + 1 7 1 ( 3 × 1 0 1 5 − 2 ) = 1 7 1 ( 2 0 × 1 0 1 5 − 2 ) = 1 7 2 ( 1 0 1 6 − 1 ) which makes the answer 1 7 .