(xâˆ’1)(1âˆ’y)=?

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Vincen: Legendary Member; Posts: 2898; Joined: Thu Sep 07, 2017 2:49 pmFollowed by: 5 members; Thanked: 6 times

by Vincen » Wed Jan 24, 2018 8:36 am

(xâˆ’1)(1âˆ’y)=?

(1) x+y=xy

(2) xâˆ’y=0

The OA is the option A.

How can I conclude that the correct answer is A? I think it should be B. Experts, can you help me? Thanks in advanced.

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Source: Beat The GMAT — Data Sufficiency | Wed Jan 24, 2018 8:36 am

DrMaths: Senior | Next Rank: 100 Posts; Posts: 82; Joined: Mon Jan 15, 2018 2:01 am

by DrMaths » Thu Jan 25, 2018 7:32 am

(xâˆ’1)(1âˆ’y)=-(x-1)(y-1) = -(xy - x - y + 1) = 1 - xy + x + y

(1) Substitute x+y=xy to get 1 - xy + xy = 1 SUFFICIENT
(2) Substitute x=y to get 1 - x^2 + 2x INSUFFICIENT

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