Q24 - Many homeowners regularly....

[smiller]

Question Type:
Inference

Stimulus Breakdown:
The stimulus states that "many" homeowners use commercial fertilizers. How many? We don't know. All we know for certain is that some do. Next we're told about "widely available" commercial fertilizers. What does "widely available" actually mean? Does it mean "most?" Not necessarily. We just know that some commercial fertilizers only contain macronutrients. In fact, we don't even know if these "widely available" ones are the same ones that "many" homeowners use. Nothing in the stimulus guarantees that these have to be the same. However, we do know that all "widely available" commercial fertilizers contain only macronutrients.

We also know that the widely available commercial fertilizers won't keep lawn soil healthy in the long term, because they only contain macronutrients, not micronutrients. Finally, we know that if grass clippings are raked, certain micronutrients are depleted from the soil.

We can summarize the stimulus using two quantified statements and three conditional statements:

homeowners < some > use commercial fertilizers
commercial fertilizers < some > contain only macronutrients

widely available comm. fertilizers → contain only macronutrients
lawn soil healthy long term → macronutrients and micronutrients
grass clippings raked → micronutrients depleted

Answer Anticipation:
The information in the stimulus must prove that the correct answer is true. Incorrect answers might involve reversed logic, or suggest a connection between the "some" statements in the stimulus when that connection isn't actually guaranteed.

Correct Answer:
(C)

Answer Choice Analysis:
(A) Unsupported. We only know that "widely available commercial fertilizers" contain only one type of nutrient. There might be other fertilizers that contain both.

(B) Unsupported. Some homeowners use commercial fertilizers, and some commercial fertilizers contain these nutrients, but homeowners might have access to these nutrients through other sources.

(C) Correct. This can be inferred from the three conditional statements:
grass clippings raked → micronutrients depleted

widely available comm. fertilizers → contain only macronutrients

~ macronutrients OR ~ micronutrients → ~ lawn soil healthy long term.

(D) Unsupported. We know that soil requires the presence macronutrients to remain healthy, but nothing in the stimulus states that soil can only get these macronutrients from commercial fertilizers. There might be other ways for the soil to receive these nutrients. There's also a detail creep in (D): the stimulus states that the nutrients must be present, but never states that "regular addition" is required.

(E) Unsupported. Homeowners who rake their grass clippings might be removing a source of nutrients, but they might be able to add nutrients to keep the soil healthy. Nothing in the stimulus prevents this from being true.

Takeaway/Pattern: A solid understanding of conditional logic and quantified logic is extremely helpful when tackling many "must be true" Inference questions. If you have a free Manhattan Prep account, download our flashcards to build you conditional logic and quantifier skills.

#officialexplanation

[wizzard880]

I got this question correct but had a hard time diagramming it and choosing between C and E. I eventually choose C but would still like to hear why E is wrong and if someone could give a diagram of this stim.

[rinagoldfield]

Thanks for your post!

This is an inference question, so let's go over what we know. We can diagram this stimulus conditionally, but note that we don't have to. The only true conditional statement here is the last sentence.

We can break that down into three statements:

1. Healthy lawn soil --> contains enough macronutrients
2. Healthy lawn soil --> contains enough micronutrients

and

3. Grass clippings raked --> micronutrients deplete

Note that Statement 3 includes the possibility that the depletion could be replenished. All we know is that the micronutrients will at least temporarily go down. Think of it like this:

I eat Thanksgiving dinner --> I gain weight

The above scenario doesn’t mean I get fatter and fatter with each Thanksgiving that passes. It could be true that I gain a little weight after the holidays, but then I work out or eat a bunch of salad and lose the weight.

Ok, back to the scenario.

We know that unraked grass clippings are one source of micronutrients. We also know that commercial fertilizers are one source of macronutrients from the earlier part of the stimulus. (Keep those micro and macro nutrients straight!)

(C) is supported. Let’s look at the contrapositive of Statement 1:

not enough macronutrients --> unhealthy lawn soil.

We know that commercial fertilizers, which provide macronutrients, are never sufficient for a healthy lawn soil according to the stimulus; "enough macronutrients" is not a sufficient condition here. We’d have to negate the logic to make it so.

We additionally know from Statement 2 that:

not enough micronutrients --> unhealthy lawn soil.

(C) presents a scenario in which micronutrients have been depleted (no lawn clippings). In order to achieve healthy lawn soil, we would need a new source of micronutrients. Yet commercial fertilizers provide no such thing. So we would need something in addition to commercial fertilizers to get our lawn healthy.

(E) is unsupported, but I see why it’s seductive! While lawns require micronutrients, there might be sources of micronutrients other than lawn clippings. Yes, the micronutrients will go down because the homeowner rakes his lawn, but perhaps the homeowner can replenish the depleted micronutrients with compost, manure, whatever.

(A) is unsupported. We only know about commercial fertilizers. Perhaps a non-commercial fertilizer can provide both macro and micro nutrients.

(B) is similar to (A). Maybe a non-commercial fertilizer can also provide these things.

(D) presumes that there are no sources of macro and micronutrients other than commercial fertilizers and lawn clippings.

Hope that helps! Write in if you have further questions.

[ttunden]

I was between C and E and picked E during the test. Now in review I understand why I was wrong.

So, during the test I picked E because I reasoned that it was proven by the contrapositive.
However, my understanding was incorrect. Apparently when micronutrients are depleted, they are temporarily eliminated, meaning they can be restored. I interpreted depleted to mean permanently eliminated.

If you interpreted it incorrectly like I did then E would have been tempting/ the choice you picked. But since it is temporarily depleted E is not true or cannot be a "MBT."

C has to be true because we know that "widely available commercial fertilizers", as stated in the stimulus, only provide macronutrients. We know that health lawn soil requires macro AND micronutrients. Also this answer choice says grass clipping ARE NOT allowed to decay and return so that eliminates the temporary depletion and makes it a permanent depletion of micronutrients. Therefore this answer choice has to be 100% true.

[magic.marshall]

Initially, I ended up going with a wrong answer choice. Upon review here is what I found.

Statements:
1. fertilizer contains essential macronutrients and does not provide essential micronutrients. (adds macro.)
2. micronutrients ... are depleted when grass clippings are raked up rather than allowed to decay and return to soil. (removes micro.)
3. home owners maintain a healthy soil long term using both macronutrients & micronutrients.
4. many home owners regularly add fertilizer to maintain a healthy balance of nutrients in soil. (adding macro only.... hey what about micro?)
(3 & 4 seem to contradict each other at face value. We know they are both true so we must infer to make them work.)
Home owners have a healthy lawn (contains micro & macro) and maintain it by adding fertilizer (add more macro) and by not removing grass clippings (do not remove micro).

Resolving Inferences:
Infer from 4. The home owners soil starts out healthy. (meaning it already begins with both macro and micro nutrients, all we need is to just maintain them.)
Infer from 2. If grass clippings are left on the lawn to decay then they allow the essential micronutrients to return to the soil. (the already provided micros are maintained.)

Break Down:
maintain healthy soil = add macro + do not remove micro.
macro = macronutrients = widely available commercial fertilizers (sufficient)
micro = micronutrients = grass clippings (sufficient)

Inferences:
Do not maintain healthy soil if add macro and remove micro.
Do not maintain healthy soil if do not add macro and do not remove micro
Not inferences:
if do not add macro then Do not maintain healthy soil (no inference)
if remove micro then Do not maintain healthy soil (no inference)
Answer Explanations:
A: If maintain healthy soil Than not just add one fertilizer. (no inference about all fertilizers or whether this is possible.)
B: If home owner Than add macro must be fertilizer. (We cannot infer this, we don't know if fertilizer is the only source of macros.)
C: If remove micro Than add macro cannot maintain healthy soil. (correct)
D: If maintain healthy soil Than must add macro and add micro. (do not need to add any micro)
E: If remove micro Than do not maintain healthy soil. (no inference about healthy soil without both macro and micro.)

hope this helps