PT3.S2.Q12 - photovoltaic power plants

vrendonvasquezvrendonvasquez Alum Member
edited November 2017 in Logical Reasoning 276 karma

I'm still trying to wrap my head around this question. I originally chose (C) thinking that it would close the gap between premises and conclusion, but realize now that it doesn't address the issue of cost. Can someone please provide a number example and corresponding explanation for (D)? Thanks!!

Comments

  • LSATcantwinLSATcantwin Alum Member Sage
    edited November 2017 13286 karma

    This is one of those "math" questions...but it's also a SA.

    Okay I always start by identifying my conclusion. It's just good practice and what J.Y. tells us to do in the CC.

    Conclusion;

    • Photovoltic power (PP) plants offer a less expensive approach to meeting demand for electricity than do traditional power plants.

    Okay cool. So what support was given?

    P1.) Because of an advance in technology the cost of PP is 1/10th of what it was 20 years ago.

    P2.) The cost of traditional power (TP) has increased in the past 20 years.

    Okay this sounds good. What do we absolutely have to have in order for our conclusion to be VALID. Remember on SA questions, when we add the correct answer choice, the argument goes to the level of a valid.

    Well we know PP got cheaper and that TP got more expensive, but we don't have any way to know what the price of each is in comparison to each other do we?

    Answer Choice D;

    • Twenty years ago, the cost of producing electric power at a PP was less than 10 times the cost of producing power at a TP.

    Before we even get to the math, do you see how this AC links the cost of PP to TP? We now have the ability to see how the prices compare to each other right? This AC has given us the mechanism we are looking for. You could technically solve this problem right here without going into the math. You can see that it is filling the gap by creating a comparison of TP prices and PP prices.

    Now for the math...

    Answer choice D says - Twenty years ago the cost of PP was less than 10 times the cost of TP.

    So if TP was $10 20 years ago, and PP was less than 10 times the cost, then the most that PP could cost is $99.99 or less. (10x10 - .01)

    Okay lets say PP was $99.99 20 years ago. It is now 1/10th the price.

    1/10th of $99 is $9.99.

    We also know that TP got more expensive than it was 20 years ago. So it moved from $10 to $11.

    $11 TP > $9.99 PP

    Do you see how the math forces PP to be cheaper than TP making our conclusion valid?

  • BinghamtonDaveBinghamtonDave Yearly Member 🍌🍌
    8668 karma

    This is an excellent form to know. It is something entirely relevant to your future studies. I believe there are two key takeaways from this problem:
    1.There is an almost dishonest assumption here by the argument's author.
    2.There is a bit of math here that I would say is early high school level of abstraction.

    So here is what we are told by this argument in the form of numbers. Here is what the argument must look like for the argument to be valid. I am constructing the argument as follows with the sufficient assumption already built in to make the reasoning as clear as I can.

    -We have these special power plants and as a result of technological innovation, they were $89 twenty years ago and $8.90 today. "One Tenth"
    -Our regular power plants have increased from $9 to $11 in that time period. "Has increased"
    -Thus, the special plants are less expensive.

    The above is a really specific story. A story that the argument has assumed holds true. Now lets take a look at a specific story that follows the facts of the argument, but the conclusion does not follow.

    -We have these special power plants and as a result of technological innovation, they were $10,000,000 twenty years ago and $1,000,000 today. "One Tenth"
    -Our regular power plants have increased from $9 to $11 in that time period. "Has increased"

    -Thus, the special plants are less expensive.

    The above story follows the facts: it is factually accurate, but the conclusion simply does not follow.

    So what (D) is doing here is telling us that our original price for the special power plant was not in the second example above. (D) is telling us that our original price for the special power plant (before it went down to 1 tenth of that price) was not more than 10 times the original price of the regular power plant. Because if it was, then the drop to 1 tenth of its original number would not prove our conclusion. Lets try to run some numbers I'm going to run 2 sets of numbers that go along with (D) and 2 sets of numbers that do not go along with (D)

    Twenty years ago:
    Special: $100
    Regular: $11

    Change in time:
    Special goes down to 1/10 of original: new price $10
    Regular increase:New price $11.21

    Twenty years ago:
    Special: $1,000
    Regular: $250

    Change in time:
    Special goes down to 1/10 of original: new price $100
    Regular increase:New price $275


    Examples that do not go along with (D):

    Twenty years ago:
    Special: $1,000
    Regular: $4

    Change in time:
    Special goes down to 1/10 of original: new price $100
    Regular increase:New price $4.01
    Conclusion does not follow

    Twenty years ago:
    Special: $40,000,000
    Regular: $1.28

    Change in time:
    Special goes down to 1/10 of original: new price $4,000,000
    Regular increase:New price $3.00
    Conclusion does not follow.

    Note here, the argument never tells us how much the regular increased by. This is important. All we know is that the regular increased. Giving the argument answer choice (D), we position the math so that any increase small or big at the end of the day for the regular is going to position the regular technology as more expensive than the special technology.

    I hope this helps,
    David

  • BinghamtonDaveBinghamtonDave Yearly Member 🍌🍌
    8668 karma

    Not here that :"Early high school level of abstraction" in the math here is meant to signify that at its core the math here is simple, but obscured by the passage itself. All we are really doing here is dividing something by 10 and making sure that number is less than another number.

    David

  • LSATcantwinLSATcantwin Alum Member Sage
    13286 karma

    Listen to @BinghamtonDave that man should be a teacher. He has a knack for breaking down LSAT questions in very clear and concise ways Haha

  • vrendonvasquezvrendonvasquez Alum Member
    276 karma

    Wow...this was just what I needed! Thank you so much @LSATcantwin and @BinghamtonDave . I now fully understand the line of reasoning behind this SA question. Do you guys have any general suggestions on how to approach this type of SA question? Or maybe additional examples of this type that involve math?

    If not, no problem. This was the first time I posted a test question, and got amazing feedback. :)

  • Pretzel LogicPretzel Logic Monthly Member
    226 karma

    @BinghamtonDave Go Senators!

  • jknarf513jknarf513 Member
    189 karma

    I know this is an old thread, but looking for some input:
    Before even getting to the ACs, I knew I needed something that helped ensure that the actual cost of the TP was in fact currently higher than the TP. This AC D was the only one that addressed that, so I picked it and moved on. Somehow in BR, though, I got all turned around and thought it was saying the opposite (That TP was <PPP) and I changed my answer.
    I could totally see myself making a reading error under time and changing it under timed conditions, too.
    Any thoughts on how common it is that we would see a trap AC that did the same thing as D but in reverse? Or is it a generally safe rule to just sort of blindly choose that bridging AC without totally working out the math? #help

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