If (x + 1)y = 4, what is the value of x ? (1) xy = 2 (2) y > 0

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Since we have 2 variables (x and y) and 1 equation, D is most likely to be the answer. So, we should consider each condition on its own first.
Actually, since only condition 1) has an equation and condition 2) does not, A is most likely the answer

Condition 1)
Since xy = 2 from condition 1), we have (x+1)y = xy + y = 2 + y = 4 or y = 2.
Then we have x = 1.
Since condition 1) yields a unique solution, it is sufficient.

Condition 2)

If y = 1, then we have x = 3.
If y = 2, then we have x = 1.
Since condition 2) does not yield a unique solution, it is not sufficient.

Therefore, A is the answer.

If the original condition includes “1 variable”, or “2 variables and 1 equation”, or “3 variables and 2 equations” etc., one more equation is required to answer the question. If each of conditions 1) and 2) provide an additional equation, there is a 59% chance that D is the answer, a 38% chance that A or B is the answer, and a 3% chance that the answer is C or E. Thus, answer D (conditions 1) and 2), when applied separately, are sufficient to answer the question) is most likely, but there may be cases where the answer is A,B,C or E.

Asked: If (x + 1)y = 4, what is the value of x ?
xy + y = 4

(1) xy = 2
2 + y = 4
y = 2
x = y/2 = 2/2 =1
SUFFICIENT

(2) y > 0
x = 4/y -1 ; x will change with change in value of y
NOT SUFFICIENT

IMO A