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(x−1)(1−y)=?

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Source: — Data Sufficiency |

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by Jay@ManhattanReview » Wed Nov 01, 2017 8:19 pm
swerve wrote:$$(x−1)(1−y)=?$$
$$(1)\ x+y=xy$$
$$(2)\ x−y=0$$

The OA is A.

I don't have clear this DS question. Please, can any expert help me with it? Thanks.
$$(x−1)(1−y)=?$$
$$x−1−xy+y =\ ? ---(1)$$
$$(1)\ x+y=xy$$
$$=> (1)\ x+y-xy = 0 ---(2)$$
Plugging in the value of (2) in (1), we get
$$0 −1 =\ -1 ---(1)$$. Sufficient
$$(2)\ x−y =\ 0$$
=> x = y

It can't help it. Insufficient.

The correct answer: A

Hope this helps!

-Jay
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