Photovoltaic power plants produce electricity from sunlight. As a resu

Negating Option D :
Twenty years ago, the cost of producing electric power at photovoltaic plants was at least 10 times the cost of traditional power.

Now consider,

Cost of Photo Plant 20 years ago = P
Cost of Traditional Plant 10 years ago = T
Now, P >= 20 T
Suppose P= 25T
Present Photo Plant Cost = P/10 =2.5T
Since Traditional Plant cost also increased , but stimulus doesnt mention the increase , let us suppose that T at present is 100T
2.5Tis not less than 100T . This doesnot break down the conclusion

Assumption negation test failed .

nightblade354 Useful contribution but I still have following in the explanation provided. I posting the explanation here for continuity in the forum discussion.


On a number of occasions the correct answer to a Justify question has been something that is also an Assumption of the argument (and vice versa). The difference is that Justify answers can also contain elements that are not necessary to the argument. It just depends on what the test makers decide to do.

Turning to the problem at hand, this is one that I've always liked, primarily because I feel as though it's a great example of how topic can influence our analysis. Most students, when they see "photovoltaic power plants," kind of go into a fugue state and just disconnect from the problem. I used to joke in class that if this question was about cheeseburgers or diamonds (things that people are familiar with), everyone would get it right. Let's analogize it and see:


Kobe beef has traditionally been one of the most expensive forms of protein in the world. As a result of advancements in farming and production, Kobe beef now costs 1/10 of what it did 20 years ago. Traditional cheeseburgers, on the other hand, have become slightly more expensive. Thus, Kobe beef is now a cheaper way to get protein than a traditional cheeseburger.

Or, let's say it was about diamonds:


Diamonds have traditionally been one of the most expensive gems in the world. As a result of advancements in production, diamonds now costs 1/10 of what they did 20 years ago. Cubic zirconia, a synthetic form of diamond, on the other hand, have become slightly more expensive. Thus, diamonds are now cheaper than cubic zirconia.

These two examples may make the problem clearer: even though the expensive item has dropped in price, that doesn't now mean it is less expensive than the originally cheaper alternative. If a diamond used to cost $1000 and a cubic zirconia cost $10, even when the diamond is 1/10 the price ($100), it is still more expensive than the cubic zirconia.

In this light, answer choice (D) suddenly makes a lot more sense, because it addresses the initial price difference, and shows that when you drop the price to 1/10, it would be lower than the other item.

This last statement does not agree with numbers mentioned above in the explanation.

i.e. explanation does not make choice D) clear winner.......

Will like to have your take on this....

Harshgmat,

That is the point. Justify questions are more like strengthen/weaken questions. We just want to fill the missing piece of information. In this example, the missing information was original cost. We do not care about future cost because the question is exclusively about the present. Is it a perfect question? Nope, as your math shows. But (D) is the best answer given the circumstances.

Still I am finding it what I will say a bitter pill...

I will need some more time with such type of questions...

Thanks nightblade354 for help.

OFFICIAL ANSWER:

As the argument is stated, there is a logical gap between the information given in the premises and the claim made in the conclusion:

Premise 1: The cost of producing electric power at photovoltaic power plants is one-tenth of what it was 20 years ago.

Premise 2: The corresponding cost for traditional plants has increased.

Conclusion: Photovoltaic power plants offer a less expensive approach to meeting demand for electricity than do traditional power plants.

From the fact that one cost has gone down while another has risen, it does not necessarily follow that the first is now lower than the second. In particular, if the cost of producing electric power at photovoltaic power plants twenty years ago was more than ten times the corresponding cost for traditional plants, then the fact that it is now one-tenth what it was is not sufficient to show that it is now lower than the corresponding cost for traditional plants, even though we are told in Premise 2 that the cost for traditional plants has increased. To conclude from the premises given in the argument that photovoltaic power plants now offer a less expensive approach than do traditional power plants, we need to know how the costs of the two methods of production were related 20 years ago—specifically that the cost of producing power at photovoltaic plants was less than 10 times the cost of producing it at traditional plants. (D) gives this information and is, thus, the credited response.

Response (A) is incorrect because it tells us about only one of the two costs, not about how the two were related 20 years ago. It in effect restates premise 2, and premises 1 and 2 together are not sufficient for drawing the conclusion.

Response (B) is incorrect. The amount of electricity produced by the different kinds of plants is not at issue.

Response (C) is incorrect. While it is relevant to the discussion, (C) does not provide the information about the comparative costs of the two kinds of plants 20 years ago that allows the conclusion to be properly drawn.

Response (E) is incorrect because the conclusion in the argument is about the present only. Whether or not the change described in (E) is expected to take place has no bearing on the claim in the conclusion that the one kind of plant offers a less expensive approach at present.

OPTION: D

GMATNinja please explain this question, I am not able to understand it

As per this, the option should be less than 10 times (not 20 times). the option D posted here is wrong? (that was my initial pre-thinking too, just FYI)

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