Can someone help me with the following question.

A firm has y% manufacturing capacity. It plans to increase its capacity by x%. Will it have enough capacity for next year?

1) y.(100-x) = 5040
2) y-x = 100

I would say the answer c
y-x=100, we can rewrite y=100+x, then Sub. into statement 1: (100+x)*(100-x)=5040, then to find out what's x. since x can not be negative, so use the positive x to find out what is y.

I may think about this wrong. I was thinking that if the max capacity is 100%, i.e. y = 100%, then increasing it by any x% will not be possible because y is 100% and capacity can't go above that.
B says that the difference between the two is 100, so it implies that y% is always greater than 100% (we can consider that y% is 120%, and x% is 20% or y% is 130 and x is 30% so the difference is always 100).
If B says that y% is greater than 100%, no matter what x% is, there will not be enough capacity. So I picked B.

Am I thinking completely wrong on this?

yeah, i'm with you here. the difference is that i'd restrict the interpretation to sensible percentages, meaning that i wouldn't allow ridiculous numbers such as 120% of capacity.
with the problem as written, this would mean that the sole sensible interpretation of statement 2 is
y = 100
x = 0
y can't be any greater (that's the meaning of "capacity"!), and x can't be less (unless you're allowing a decrease to be masked as a "negative increase", something i'm 100% positive that they wouldn't do).
so i guess that would be sufficient.

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there's no way this problem, as written, can come from the gmatprep software. some of the official problems are weird, but the writing is always crystal clear - no ambiguity or vagueness, ever. so the original poster has either misattributed the source or butchered the original version of the problem.

what do you say, original poster?