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 Doubtful Divisibility (Posted on 2012-05-09)
For two positive integers x and y, we have:
(100*x + y)/(x+y) = 1 + (99*x)/(x+y) and,
(100*x + y)/(x+y) = 100-(99*y)/(x+y) . . . . (i)

Since both x and y are positive, we have: x+y > x and x+y > y

Accordingly, x+y does not divide either x or y, and, consequently from (i) we deduce that x+y divides (100*x + y) whenever, (x+y) divides 99.

Now, substituting (x, y) = (20, 16), we have: x+y = 36 which does not divide 99 and consequently we can conclude that 36 does not divide 2016.

However, in reality 2016/36 = 56 and, accordingly the above result is flawed.

Can you spot the error?

 No Solution Yet Submitted by K Sengupta No Rating

Comments: ( Back to comment list | You must be logged in to post comments.)
 re: Error(s) spotted Comment 3 of 3 |
(In reply to Error(s) spotted by Jer)

If (x+y) divides 99, then it also divides 99*x, and satisfies your criterion, so the deduction from (i) is indeed true, but its converse is not.
 Posted by Charlie on 2012-05-09 13:42:55

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