Egyptian Fraction with New Years 2015

How many ordered pairs of positive integers ( x , y ) (x,y) are there such that

1 x + 1 y = 1 2015 ? \frac{ 1}{x} + \frac{ 1}{y} = \frac{1}{2015}?


The answer is 27.

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2 solutions

Rewrite the equation as

x + y x y = 1 2015 x y 2015 x 2015 y = 0 \dfrac{x + y}{xy} = \dfrac{1}{2015} \Longrightarrow xy - 2015x - 2015y = 0

( x 2015 ) ( y 2015 ) = 201 5 2 \Longrightarrow (x - 2015)(y - 2015) = 2015^{2} .

Now 2015 = 5 13 31 2015 = 5*13*31 , and so 201 5 2 = 5 2 1 3 2 3 1 2 2015^{2} = 5^{2}*13^{2}*31^{2} , which has ( 2 + 1 ) ( 2 + 1 ) ( 2 + 1 ) = 27 (2 + 1)(2 + 1)(2 + 1) = 27 divisors. Now each of these divisors corresponds to a distinct solution pair ( ( x 2015 ) , ( y 2015 ) ) ((x - 2015), (y - 2015)) , which in turn corresponds to a distinct solution pair ( x , y ) (x,y) . Thus there are 27 \boxed{27} solution pairs.

Nice solution. I know a little of Number Theory to understand your steps.

Niranjan Khanderia - 6 years, 6 months ago

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Thanks. This is the standard approach to this type of problem; it's an application of what is known as Simon's Favorite Factoring Trick, (SFFT) .

Brian Charlesworth - 6 years, 5 months ago

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Nice Solution but please tell me how SFFT is used?

Tarun Chahal - 6 years, 5 months ago
Paola Ramírez
Jan 17, 2015

Use Simon's Favorite Factoring Trick

( x 2015 ) ( y 2015 ) = 201 5 2 (x-2015)(y-2015)=2015^2

As 201 5 2 = 3 1 2 × 1 3 2 × 5 2 2015^2=31^2\times13^2\times5^2 \therefore number of divisors of 2015 2015 is 3 × 3 × 3 = 27 3\times3\times3=\boxed{27}

Did the same! :D

Samanvay Vajpayee - 5 years, 11 months ago

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