(A) if he could sell more than 400 sandwiches, it is likely that this would help him break even.

(B) CORRECT.

(C) this statement does not tell us that Adam will have fewer than 400 or that his plan cannot work.

(D) Out of scope..

(E) If he shares the cost of the booth rental, then he is more likely to sell
enough sandwiches to cover his costs.
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A: does not weaken
C: we don't know : >400 => strengthens / < 400 => weakens
D: does not deal with the subject
E: does not weaken

=> B (raw materials + labor increase global costs while they were not taken into account by Adam)

clearly B - if his cost turns out to be 1000 and he sells exactly 400 for 10* = 4000
therefore, 4000-1000 = 3000. he is under a loss.

The wording of the question is what lets us make a clear choice here (B)--you're absolutely correct prab in pointing out that we don't have any information given about the initial costs. But the question asks for which choice "casts the most doubt" -- not disproves Adam's claim.

B in no way disproves Adam's claim about what he needs to sell to break even. But it directly addresses an assumption that underlies the argument as it is given. The other choices are "less good"for the reasons listed by the posters above-- that doesn't mean B is good; it just means B is the best of the flawed bunch.

Conclusion: "in order to cover his cost"

Cost includes the rent for the booth AND the materials/labors to make the sandwiches.

Lets suppose he sold 400 sandwiches to earn $4000, which will only cover the cost of renting the booth but not the cost of the materials/labors. Thus, B is correct.

Posted from GMAT ToolKit

Adam concludes that to cover the $4,000 cost of a booth at the fair, he will need to sell 400 sandwiches at $10 each. He relies on information about the number of customers served by the average booth in previous years. The question asks what most weakens Adam’s conclusion. The correct answer will either show that Adam has overlooked other important costs, or that his logic is fundamentally flawed. There could be more than one choice that seems to suggest that Adam might not break even; the correct answer will be the one that eliminates any possibility that he will break even.

(A) Though this is certainly true in a practical sense, it does not show that Adam's plan will not work and that he will not stay within his budget. In fact, if he could sell more than 400 sandwiches, it is likely that this would help him break even.
(B) CORRECT. Each sandwich costs some amount of money to make and sell, so even if Adam sells the 400 sandwiches as planned, his net income will certainly be less than $4,000. Based on this statement, Adam definitely will not break even.
(C) Though it is true that an average number of customers means some booths served more and some served fewer, this statement does not tell us that Adam will have fewer than 400 or that his plan cannot work. Consider also that each customer might buy more than one sandwich, so Adam could have fewer customers but still sell enough sandwiches to cover his costs.
(D) Although some people will buy other types of food at the fair, this statement fails to conclusively weaken Adam’s logic. Adam relied on information about the average number of customers for food booths in previous years, but those booths may have sold sandwiches or other foods – we simply don’t have enough information to judge whether this statement strengthens or weakens Adam’s conclusion.
(E) Sharing the booth would actually make it more likely that Adam would break even. If he shares the cost of the booth rental, then he is more likely to sell enough sandwiches to cover his costs.
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Adam will spend $4,000 to rent a booth at the town’s annual county fair to promote his new Sandwich Shoppe. In previous years, the average food booth at the fair served 400 customers. Thus, in order to cover his costs, Adam has calculated that he must sell 400 sandwiches for $10 each.

Which of the following statements casts the most doubt on Adam’s chances of breaking even at the county fair?

A. Adam should not limit himself to 400 sandwiches; he should be prepared to sell more. - Irrelevant
B. Adam has not factored in the costs of raw materials or labor in calculating the amount of revenue he needs to generate to break even. - Correct - Adam only factors in rental cost but does not considers any other cost
C. If booths in previous years averaged 400 customers, that means some booths served more than 400 and some served fewer than 400. - Irrelevant
D. Many people prefer other types of food to sandwiches. - Irrelevant
E. If Adam shares his booth, and the cost of the rental, with a complementary business, he will not have to sell as many sandwiches to break even. Irrelevant -

Answer B
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BREAK EVEN means recovering the amount spent.
This includes: raw materials+labor+electricity+rent etc
Adam has taken into account only the rent, but he should factor in the other factors as well.
Answer is clearly B.

OFFICIAL EXPLANATION

Adam concludes that to cover the $4,000 cost of a booth at the fair, he will need to sell 400 sandwiches at $10 each. He relies on information about the number of customers served by the average booth in previous years. The question asks what most weakens Adam’s conclusion. The correct answer will either show that

Adam has overlooked other important costs, or that his logic is fundamentally flawed. There could be more than one choice that seems to suggest that Adam might not break even; the correct answer will be the one that eliminates any possibility that he will break even.

(A) Though this is certainly true in a practical sense, it does not show that Adam's plan will not work and that he will not stay within his budget. In fact, if he could sell more than 400 sandwiches, it is likely that this would help him break even.

(B) CORRECT. Each sandwich costs some amount of money to make and sell, so even if Adam sells the 400 sandwiches as planned, his net income will certainly be less than $4,000. Based on this statement, Adam definitely will not break even.

(C) Though it is true that an average number of customers means some booths served more and some served fewer, this statement does not tell us that Adam will have fewer than 400 or that his plan cannot work. Consider also that each customer might buy more than one sandwich, so Adam could have fewer customers but still sell enough sandwiches to cover his costs.

(D) Although some people will buy other types of food at the fair, this statement fails to conclusively weaken Adam’s logic. Adam relied on information about the average number of customers for food booths in previous years, but those booths may have sold sandwiches or other foods – we simply don’t have enough information to judge whether this statement strengthens or weakens Adam’s conclusion.

(E) Sharing the booth would actually make it more likely that Adam would break even. If he shares the cost of the booth rental, then he is more likely to sell enough sandwiches to cover his costs.
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