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xy and yx are a pair of reversed two digit positive integers. If x^2-y

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Joined: 07 Dec 2014
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xy and yx are a pair of reversed two digit positive integers. If x^2-y  [#permalink]

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New post 26 Aug 2018, 14:03
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Difficulty:

  85% (hard)

Question Stats:

44% (02:27) correct 56% (02:11) wrong based on 43 sessions

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xy and yx are a pair of reversed two digit positive integers. If x^2-y^2=45, how many such pairs are there?

A. 0
B. 1
C. 2
D. 3
E. 4
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Joined: 22 May 2016
Posts: 2117
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xy and yx are a pair of reversed two digit positive integers. If x^2-y  [#permalink]

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New post 26 Aug 2018, 17:44
1
gracie wrote:
xy and yx are a pair of reversed two digit positive integers. If x^2-y^2=45, how many such pairs are there?

A. 0
B. 1
C. 2
D. 3
E. 4

If \(x^2-y^2=45\), then
\((x+y)(x-y)=45\)

\((x+y)=a\): one factor of 45
\((x-y)=b\): another factor of 45

Factors of 45
\((a)*(b)=45\)
\(1*45=45\)
\(3*15=45\)
\(5*9=45\)

The first set of factors will not work
\(x+y=45\)
\(x-y=1\) Add
\(2x=46\)
\(x=23\)

Second set of factors
\(x+y=15\)
\(x-y=3\) Add
\(2x=18\)
\(x=9\)

\(x-y=3\)
\(9-y=3\)
\(y=6\)

One pair, xy and yx: 96 and 69
\((x^2-y^2)=(9^2-6^2)=(81-36)=45\)

Third set of factors
\(x+y=9\)
\(x-y=5\) Add
\(2x=14\)
\(x=7\)

\(x-y=5\)
\(7-y=5\)
\(y=2\)

Another pair: 72 and 27
\((x^2-y^2)=(7^2-2^2)=(49-4)=45\)

There are no more factor sets.

xy and yx are
96 and 69
72 and 27

Number of pairs: 2

Answer C
GMAT Club Bot
xy and yx are a pair of reversed two digit positive integers. If x^2-y &nbs [#permalink] 26 Aug 2018, 17:44
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xy and yx are a pair of reversed two digit positive integers. If x^2-y

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