Logical ReasoningDifficulty: Hard

PT3 S2 Q12 Explanation

Photovoltaic power plants produce electricity

A free, expert breakdown of this official LSAT Logical Reasoning question.

TopicsSufficient Assumption

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Stimulus

Photovoltaic power plants produce electricity from sunlight. As a result of astonishing recent technological advances, the cost of producing electric power at photovoltaic power plants, allowing for both construction and operating costs, is one-tenth of what it was 20 years ago, whereas the corresponding cost for traditional plants, which burn fossil fuels, approach to meeting demand for electricity than do traditional power plants.

What this question is testing

Sufficient Assumption

Premise

The argument tells us PV costs are one-tenth of what they were 20 years ago, and traditional costs have gone up.

Conclusion

So PV is now the cheaper option.

Evaluate

This sounds reasonable, but watch the gap. We are told how each cost has changed, but not what they started at. Without that, the comparison does not work.

Imagine PV used to cost $100 per unit and traditional used to cost $1. Now PV is at $10 (one-tenth of $100) and traditional has risen to, say, $2. PV is still way more expensive. The "one-tenth" sounds like a huge improvement, but it does not necessarily put PV below traditional.

What we need: PV's starting cost has to have been close enough to traditional's starting cost that, after the one-tenth drop, it actually undercuts the new traditional cost. The simplest version: 20 years ago, PV was less than 10 times what traditional cost. Then one-tenth of PV's old cost is less than traditional's old cost — and since traditional has gone up, PV is still less.

Goal

Find the answer that locks in the starting ratio: 20 years ago, PV was less than 10 times the cost of traditional.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question
12.

The conclusion of the argument is properly drawn if which one of the

Answer choices

  1. Premise Support7% picked this

    The cost of producing electric power at traditional plants has increased over the

    This restates the premise about traditional plant costs increasing. Restating a premise does not bridge the gap to the conclusion. We need a baseline starting ratio to compare current PV cost to current traditional cost; this answer does not provide one.

  2. Irrelevant Quality2% picked this

    Twenty years ago, traditional power plants were producing 10 times more electric power than

    This is about the relative output (how much electric power) of the two plant types 20 years ago, not about the relative cost. The argument is comparing per-unit production cost, not total volume produced. Volume of output does not determine which is cheaper today.

  3. Irrelevant Relationship19% picked this

    None of the recent technological advances in producing electric power at photovoltaic plants can be applied to producing

    Whether PV technology can be applied to traditional plants does not address the comparison the conclusion needs. The conclusion compares current PV cost to current traditional cost; transferability of technology does not affect that comparison. Even if no PV advances apply to traditional plants, we still cannot conclude PV is cheaper without knowing the starting ratio.

  4. Correct61% picked this

    Twenty years ago, the cost of producing electric power at photovoltaic plants was less than 10 times the cost of

    Why this is right

    This is the missing baseline. If 20 years ago PV cost was less than 10 times traditional cost, then PV's current cost (one-tenth of its old cost) is less than traditional's old cost — and since traditional cost has only gone up since then, PV's current cost is also less than traditional's current cost. That is exactly the conclusion. Without this assumption, the "one-tenth" reduction might leave PV still above traditional today.

    Skill tested: Sufficient Assumption · how this choice captures the argument's function is the move to repeat next time.

  5. Out of Scope11% picked this

    The cost of producing electric power at photovoltaic plants is expected to decrease further, while the cost of producing power at traditional

    Future trends do not establish the current cost comparison. The conclusion is about which is less expensive now, not which is expected to be less expensive later. Without the starting ratio, expected future trends cannot bridge the gap.

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