Question

Many gardeners believe that the variety of clematis vine that is most popular among gardeners in North America is jackmanii. This belief is apparently correct since, of the one million clematis plants sold per year by the largest clematis nursery in North America, ten percent are jackmanii.

Which of the following is an assumption on which the argument depends?

(Because of copyrights, the complete official question is not copied here. You can access the question here: GMAT Club)

Difficulty: Medium

Accuracy: 66%

Based on: 5786 sessions

Solution

The Story

Many gardeners believe that the variety of clematis vine that is most popular among gardeners in North America is jackmanii. The statement talks about a belief held by many gardeners. What is the belief? Jackmanii is the most popular type of clematis vine among gardeners in North America.

This belief is apparently correct since, of the one million clematis plants sold per year by the largest clematis nursery in North America, ten percent are jackmanii. The author agrees with the belief based on some evidence.

‘apparently correct’: this phrase indicates that the author accepts the belief to be correct based on some piece of information.

What is the piece of information? That 10% of the clematis vines sold by the largest nursery (out a total of 1 million clematis vines sold) are jackmannii.

Author’s logic:
10% of the large number (1 million) of clematis vines sold by the largest nursery are jackmanii (basis). Therefore, jackmannii is the most popular vine of any gardeners in North America.

Gap(s) in logic:
– What’s the big deal with 10%? The author must believe that no other clematis vine sold more than 10%. There could be a vine that constituted more the 10% of the sales.
– Smaller nurseries might overshadow the largest nursery’s sales.
– Does overall sales indicate popularity among gardeners? Maybe architects order plants for landscaping without inputs from gardeners.

There could be additional gaps in the argument. I came up with these.

Question Stem

Which of the following is an assumption on which the argument depends?

We’re looking for an assumption. ‘an assumption on which the argument depends’: this wording implies that we are looking for a statement without which the argument would not make any sense. Thus, one test we can apply while going through the answer choices is: does the argument still remain valid if we add the negation of the answer choice to the context. If it does, then the argument would not be dependent on the answer choice, and thus the choice would be incorrect. Also, an assumption is something that the author would have needed to make the argument. So, assumptions support the argument. If we find an answer choice that weakens the argument, it cannot be an assumption.

Prediction:
The author assumes that:
– 10% is the highest portion
– The sales of the largest nursery are representative of overall sales
– Popularity of a vine among gardeners can be determined on the basis of sales in nurseries

Answer choice analysis

Answer Choice: A

Correct

Selected by: 66%

What is the author’s logic?
Since 10% of the vines sold by the largest clematis nursery were jackmanii the most popular such vine is jackmanii.
When would 10% be a big deal?
When no other vine sold 10% or more.
Now, if the nursery sells 10 or fewer varieties (negation), then 10% jackmanii sales would not be the single largest proportion. So, the author’s logic falls flat.

For the author to conclude that jackmanii is the most popular vine variety among gardeners on the basis the 10% figure of the largest nursery, the author must have assumed that 10% is the highest proportion of sales by any clematis vine variety.

Let’s use a bit of math to understand why this answer choice is correct.

Say, the nursery sold 9 varieties of clematis vines. Even if each variety sold equally, the proportion of sale would be 11.11% each. If any variety sold more than the others, its proportion would be even higher than 11.11%. And if there was actually a vine which was responsible for >10% of the sales, then the author would not have concluded that jackmanii is the most popular on the basis of its 10% sales figure.

So, had there been 10 or fewer clematis vine varieties sold at the nursery, at least one variety would have sold a proportion higher than 10%. That would destroy the argument’s logic.

Why does the author believe that jackmanii is the most popular clematis vine on the basis of the 10% figure? The author must assume that 10% is the highest proportion among all clematis vines. That would only happen if the total number of vines is more than 10.

Note: I find many test-takers eliminate this answer choice in their first-go, and only come back to it after eliminating all the others. To understand why this answer choice is correct requires a deeper level of quantitative reasoning than most CR questions.

Answer Choice: B

Incorrect

Selected by: 5%

Statement: The nursery does not sell any plants other than vines.

If this statement were true, then maybe the nursery is indeed a reliable source to understand which vine is the most popular vine among gardeners. i.e., the answer does support the argument mildly. However, for the argument to hold true, is it necessary for the nursery to not sell other plants? What if it did? What if the nursery did sell plants in addition to clematis vines (negation)? Even then, as long as it is the largest clematis vine nursery in North America, the argument’s logic remains intact.

It helps to be clear that the 10% sales are of ‘the one million clematis plants sold per year’ and not of the total number of plants sold per year by the nursery.

Answer Choice: C

Incorrect

Selected by: 2%

What impact does this answer choice have on the argument? The conclusion of the argument is that that the belief that jackmanii is the most popular clematis vine among gardeners in North America is correct. If some of the jackmanii sold by the nursery are sold to gardeners outside North America, my belief in the conclusion reduces. This answer choice weakens the argument, thus cannot be an assumption.

We can also evaluate this answer choice by checking whether it is necessary for the argument. What if none of the jackmanii sold by the nursery is sold to gardeners outside North America (negation)? In that case, our belief in the argument goes up. The negation of the statement supports the argument instead of breaking it down. Thus, this answer choice is incorrect.

I find that many people reject this answer on the basis of the word ‘some’.

Let’s replace the word ‘some’ with the word ‘most’.

Most of the jackmanii sold by the nursery are sold to gardeners outside North America.

Does this change make this answer choice correct?

If most jackmanii sold by the nursery are exported outside NA, the argument gets weakened. We would then believe less in the point that jackmanii is the most popular clematis vine in NA.

If a statement weakens the argument, it can’t be an assumption. The author would not have assumed something that weakens the argument.

Thus, the reasoning that this answer choice is not an assumption because it mentions the word ‘some’ is flawed. ‘some’ or ‘most’, the answer choice weakens the argument, and thus is not an assumption on which the argument depends.

Answer Choice: D

Incorrect

Selected by: 6%

The argument is based on the nursery’s sales.

Gardeners could buy from a nursery and grow clematis in their gardens, or they could plant from scratch and grow clematis in their gardens. In the former case, I do not learn anything about categories within clematis. Thus, the statement becomes irrelevant for the argument. In the latter case, the argument actually gets weakened. If most gardeners plant clematis from scratch in their gardens, the sales data of the largest clematis nursery would not really mean much with regards to which clematis vine is the most popular among gardeners.

Thus answer choice either is irrelevant or weakens the argument. Either way, it is not necessary for the argument that most gardeners grow clematis in their gardens. Even if most didn’t (negation), the argument’s logic still remains intact.

Answer Choice: E

Incorrect

Selected by: 21%

Tricky one. This answer choice does strengthen the argument. If for all clematis-specializing-nurseries, at least 10% of the clematis plants sold are jackmanii, I certainly start to believe more that jackmanii is the most popular clematis vine variety among gardeners in North America.

Is the statement necessary though?
Is it absolutely necessary for the argument that all ‘specialist’ nurseries sell such a proportion of jackmanii?
What if even one nursery (perhaps a small one) sells a smaller proportion of jackmanii than 10% (negation)? Does the argument break down then? No, it doesn’t. The argument still makes sense even if few specialist nurseries sell less than 10% jackmanii.

Remember, we are looking for an assumption on which the argument depends.

Does the argument depend on this answer choice?
In other words, is it absolutely necessary for all such nurseries to at least 10% of their clematis sales through jackmanii?

No.

These are the kind of questions that help us understand why this answer choice is incorrect.

Additional Notes

FAQ – related to answer choice A:

Even if there are more than 10 varieties of clematis vines, it isn’t necessary that 10% is the highest share. Another vine might still have sold more. Then shouldn’t option A be wrong?

There is a flaw in this reasoning. When we are looking for an assumption, we are looking for something that is necessary for the argument to make sense – i.e., without which the argument should not make any sense. However, we are not looking for something that makes that argument logically foolproof. In other words, an assumption would be necessary for the argument, but it need not be sufficient.

The above-mentioned reasoning simply states that we may not be able to reach the conclusion even if we add option A to the argument. And that’s fine. Option A is necessary – i.e., if there are 10 or fewer varieties in the nursery, then the argument’s logic gets shattered. Option is not enough (sufficient) – i.e., even if option A is true, it is possible that the conclusion may still not follow.

This distinction between necessary and sufficient is quite helpful in dealing with assumption questions. I see test-takers make two kind of mistakes in this context:

  1. Eliminate the correct answer because it isn’t sufficient, even though it is necessary
  2. Select an incorrect answer because it is sufficient (i.e. it confirms the conclusion), even though it is not necessary

If you have any doubts regarding any part of this solution, please feel free to ask in the comments section.

Anish Passi

GMAT Coach

With over a decade of GMAT training experience, top 1 percentile scores on the CAT and GMAT, Anish is one of the most qualified GMAT coaches in India. He has founded and served as Director in two ed-tech startups in the past. An alumnus of IIM Ahmedabad and McMaster University, Canada, he strongly believes in the right way of learning, and in learning to grow and not just to get a score, an admission or a job. Anish offers private tutoring and live-online classroom courses. His sessions are often sprinkled with jokes that only he finds funny (you’ve been warned). To connect with him, write to him at a@TheGMATCo.com

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