Equation Solution

If x x and y y are positive integers, each greater than 1 1 , and if 13 ( x 1 ) = 17 ( y 1 ) 13(x-1)=17(y-1) , what is the least possible value of x + y ? x+y?


The answer is 32.

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2 solutions

Chew-Seong Cheong
Dec 28, 2017

From the given equation 13 ( x 1 ) = 17 ( y 1 ) 13(x-1) = 17(y-1) , it is obvious the equation is true when x = y = 1 x=y=1 . But x , y > 1 x, y > 1 . SInce 13 and 17 are primes, for the equation to be true, x 1 x-1 and y 1 y-1 must be the multiples of 17 and 13 respectively. Therefore the smallest x x and y y greater than 1 are when x 1 = 17 x = 18 x-1=17 \implies x = 18 and y 1 = 13 y = 14 y-1=13\implies y=14 and x + y = 18 + 14 = 32 x+y = 18+14 = \boxed{32} .

Jerry McKenzie
Dec 28, 2017

Let 13(x-1) and 17(y-1) be a product a. note that a must have a factorization including 13, and 17 since they are both prime. The leas common multiple of 13 and 17 leads us to 13=y-1, and 17=(x-1), thus y=14, x=18 and y+x=32. No smaller products can be found since the product 13*17 is the least common multiple.

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