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xyz21
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xyz21
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IanStewart
xyz21
Guys, just for fun.. this question used to be a popular hazing question in my college. I have adapted it to PS format.

===

if x and y are integers such that x not equal to y, atleast how many solutions exist for following equation:

x^y + y^x = n such that n is an integer variable belonging to [1000, 1099]

A) 100
B) 200
C) 250
D) 350
E) 450

Hope there are members from my college who would recognize this one :)

Of course, with those answer choices, A must be correct (if this is a GMAT question). Even if you find there are 940 solutions, then certainly there are 'at least 100'.

But as GMATTiger pointed out above, we have the solution x = 1, y = 1000. We actually have solutions x = 1, and y = 999, 1000, 1001, 1002, ... 1098, giving us a hundred solutions. The equation is symmetric in x and y, so if you consider the solutions y = 1, x = 999, 1000, 1001... 1098 to be different from the first set of solutions, we then get up to 200. If my mental arithmetic was right, there aren't any other solutions, though I checked that very quickly.

Bingo.

Thanks, Ian -- I should have framed the wording as "..how many solutions.." and should have avoided use of at least. You got it, those 200 form the solution set. Good show!
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IanStewart
xyz21
Guys, just for fun.. this question used to be a popular hazing question in my college. I have adapted it to PS format.

===

if x and y are integers such that x not equal to y, atleast how many solutions exist for following equation:

x^y + y^x = n such that n is an integer variable belonging to [1000, 1099]

A) 100
B) 200
C) 250
D) 350
E) 450

Hope there are members from my college who would recognize this one :)

Of course, with those answer choices, A must be correct (if this is a GMAT question). Even if you find there are 940 solutions, then certainly there are 'at least 100'.

But as GMATTiger pointed out above, we have the solution x = 1, y = 1000. We actually have solutions x = 1, and y = 999, 1000, 1001, 1002, ... 1098, giving us a hundred solutions. The equation is symmetric in x and y, so if you consider the solutions y = 1, x = 999, 1000, 1001... 1098 to be different from the first set of solutions, we then get up to 200. If my mental arithmetic was right, there aren't any other solutions, though I checked that very quickly.

Thanks Ian, you got it.

I knew that I was missing something. I pondered for a while but missed that one.....
I went for 1, 2, 3, 4, 5,.............. but not for 1001, 1002...... and 1099.



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