The more television children watch, the less competent they are in mat

This argument is overall assuming that everything is the same between South Korea and the U.S. What if South Korea spends all of its school day on math while the U.S. is more of a liberal arts type of education. Also, this argument is assuming that the same people who watch TV for 5+ hours a day are not the same people that excel in math. What if it just so happens that they do both? In addition, there is a really interesting shift from "understanding advanced measurement and geometric concepts" to "doing well in mathematics." It seems to me that this goes from a specific skill to a general knowledge

The first sentence looks like a fact, but ultimately I think LSAC intended it as a claim we could attack (since the conclusion says something similar).

In any modern test, I think the second sentence would start with "After all" / "To see this", etc., so that we would know that the first sentence is a subsidiary conclusion we can attack.

- notice the gap between adv meas / geom concepts vs. 'doing well in mathematics'.

We could have easily seen a correct answer say something like
(A) one's ability to understand advanced measurement and geometric concepts is relevant to whether one is doing well in mathematics

For (C), it doesn't literally say the same thing as the conclusion, but I agree that it feels close enough to think, "Hmmm, this doesn't seem to be addressing a gap or ruling out a potential objection".

(C) is focused on children's self-directed efforts to watch less television. It's saying that the kids who really want to do well in math will take it upon themselves to watch less TV.

The author doesn't have to assume that. He's free to make his conclusion while assuming that we will have to tear all kids kicking and screaming away from the television.

(not to nerd out here, but I'm not in love with the correct answer. If I say, "to win that tennis tournament, Ben is going to have to improve his serve", it doesn't weaken that claim at all to point out that his serve isn't the only problem: his backhand and net game are also struggling. Similarly, the fact that (E) points out a gap in instructional quality in the US vs. South Korea doesn't really affect the claim that excessive TV watching is hurting US students. More than one thing can concurrently be hurting US students. So as I said at the outset, the writing of this question is pretty loose by modern standards)

-------------*----------------*------------------
The above is an explanation by an MGMAT instructor.
In the conclusion, watching less television is a necessary condition for "United States children... well in maths", but it is not a sufficient condition.
If we change the conclusion from
Therefore, if United States children are to do well in mathematics, they must watch less television.
TO -
If US children watch less television, then they will do well in maths.
Now, since "watching less television" is a sufficient condition, so won't option E be a better assumption?

AjiteshArun , mikemcgarry ,GMATNinja ,ChiranjeevSingh , other experts -- please enlighten .

Skywalker18, I agree. If the conclusion were changed as you suggested, then choice (E) would very clearly be a required assumption.

As for the given conclusion, the word "must" is very important. The author is implying that television is the problem in the US. If the kids in the US would watch less television, then they would do well in math.

In order for this reasoning to hold, we have to assume that there are no other obstacles preventing the US children from doing well in math. But what if, for example, the instruction in those concepts is really bad in the US? In that case, reducing television watching might not be enough to cause those children to do well. The first sentence suggests that they might do a bit better, but they still might not do well.

Furthermore, if the instructional materials/methods themselves are bad, then it's possible that the children would do better if they simply had better materials/methods. Maybe they could continue watching 5 hours of television per day and still do pretty well if the instruction were improved.

The author's logic is only valid if we assume that the quality of the instruction available to US children is close to that of the instruction available to SK children. Choice (E) is a more obvious assumption if we change the conclusion, as noted by Skywalker18, but as is choice (E) is still the best answer.

I hope that helps!

DavidTutorexamPAL GMATNinja nightblade354 gmat1393 AjiteshArun Skywalker18 VeritasKarishma
Why D is wrong ?
I rejected it because E sounded more good to me but I want to know the exact reason why it is wrong

D is primarily wrong because it is not an assumption - it is the argument! .
The claim is that mathematical knowledge is hurt by TV watching; D merely restates this, in a more detailed way. But an assumption has to be separate than the claim itself.

Another reason is that, in being more specific, D is actually not essential. An assumption must be true in order for an argument to be true; in other words, it is impossible that the argument is true and the assumption isn't . this simply isn't the case here: it is clearly possible that TV hurts math knowledge in general, but that being 59 minutes or 61 minutes doesn't matter at all: in other words, that the claim in D is not true.

Try the negation method- negate each option and check whether conclusion holds or not.

If you negate the correct option, the conclusion also fails.

Example- In option E- The instruction in advanced measurement and geometric concepts available to children in the United States is not substantially worse than that available to South Korea children.

Now negate the option - The instruction in advanced measurement and geometric concepts available to children in the United States is substantially worse than that available to South Korea children.

So it tells us that USA children are not performing well because of poor teaching as compared to South Korea and not because of TVs.
So conclusion does not hold good.

So E is the correct answer.

rohanrale
Your answer seems to be the most easiest so far to understand.
Is it preferable to use the negation technique (negate the answer choice to check if the conclusion holds) to answer most of the assumption questions?
Or would you suggest something else?

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