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A compelling optical illusion called the illusion of [#permalink]

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11 Jul 2008, 07:02

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Official Guide for GMAT Verbal Review, 2nd Edition

Practice Question Question No.: 76 Page: 143 Difficulty:

A compelling optical illusion called the illusion of velocity and size makes objects appear to be moving more slowly the larger the objects are. Therefore, a motorist’s estimate of the time available for crossing a highway with a small car approaching is bound to be lower than it would be with a large truck approaching.

The conclusion above would be more properly drawn if it were made clear that the

(A) truck’s speed is assumed to be lower than the car’s (B) truck’s speed is assumed to be the same as the car’s (C) truck’s speed is assumed to be higher than the car’s (D) motorist’s estimate of time available is assumed to be more accurate with cars approaching than with trucks approaching (E) motorist’s estimate of time available is assumed to be more accurate with trucks approaching than with cars approaching

IMO B. Truck is heavier, so if car and truck run at same speed, truck will appear slower. Therefore driver from opposite side will estimate that car will quickly cross as compared to truck.

Ans : B. In all the other cases the conclusion drawn may not be true. Lets say option A was true. In this case since the truck is slower than the car it will in any case take more time to reach the motorists irrespective of other factors like illusion of velocity or size.

Premise: due to illusion - object size makes objects appear to be moving more slowly. Conclusion: time to cross car is less than time to cross truck. Hence Car takes less time while truch takes more time to cover the distance.

A. if truck's speed is less, it is not illusion - Can not be the answer B. If speeds are same, illusion acts - Answer C. again Speed should be constant D and E: Accuracy of Motorist's estimation is not be considered as we are making relation between illusion and time while speed is contant.
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Re: A compelling optical illusion called the illusion of [#permalink]

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24 Mar 2012, 18:36

I have chosen B for this question:

The optical illusion is only true if the 2 objects are moving at the same speed.

A. This is not an assumption made by the argument. If the truck's speed is slower, then what is the point of the optical illusion? Clearly, if the truck is moving slower, the motorist will have more time.

B. This is the correct answer. We have a good comparison now because we are comparing 2 objects moving at the same speed, so the larger truck will appear to be moving slower than the small car.

C. Same as A but reversed

D. How can we assume this? Nothing in the passage mentions the motorist's ability to estimate time.

A compelling optical illusion called the illusion of [#permalink]

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22 Sep 2012, 14:02

A compelling optical illusion called the illusion of velocity and size makes objects appear to be moving more slowly the larger the objects are. Therefore, a motorist’s estimate of the time available for crossing a highway with a small car approaching is bound to be lower than it would be with a large truck approaching. The conclusion above would be more properly drawn if it were made clear that the (A) truck’s speed is assumed to be lower than the car’s (B) truck’s speed is assumed to be the same as the car’s (C) truck’s speed is assumed to be higher than the car’s (D) motorist’s estimate of time available is assumed to be more accurate with cars approaching than with trucks approaching (E) motorist’s estimate of time available is assumed to be more accurate with trucks approaching than with cars approaching

(My only query - Read this after doing the Q why cannot the answer be D?. There are two things. First, either his estimate about cars is accurate (which should be if we have to prove that he has this optical illusion concerning large objects. Second, his estimate about cars is more accurate than trucks - which might mean that both his estimates are incorrect, hence he has optical illusion concerning large and small objects. Then this answer might not make sense??? --- please guide...

A compelling optical illusion called the illusion of velocity and size makes objects appear to be moving more slowly the larger the objects are. Therefore, a motorist’s estimate of the time available for crossing a highway with a small car approaching is bound to be lower than it would be with a large truck approaching. The conclusion above would be more properly drawn if it were made clear that the (A) truck’s speed is assumed to be lower than the car’s (B) truck’s speed is assumed to be the same as the car’s (C) truck’s speed is assumed to be higher than the car’s (D) motorist’s estimate of time available is assumed to be more accurate with cars approaching than with trucks approaching (E) motorist’s estimate of time available is assumed to be more accurate with trucks approaching than with cars approaching

(My only query - Read this after doing the Q why cannot the answer be D?. There are two things. First, either his estimate about cars is accurate (which should be if we have to prove that he has this optical illusion concerning large objects. Second, his estimate about cars is more accurate than trucks - which might mean that both his estimates are incorrect, hence he has optical illusion concerning large and small objects. Then this answer might not make sense??? --- please guide...

I could not really get your statement, however, I don't think the accuracy of time is really important in this question.

The premise already says that [b]larger objects tend to appear to be moving slowly.[/b] . You have to take this at Face value .

So if the truck and the car are moving at the same speed , only then will the motorist estimate that the truck is a large object and hence, he estimates it to be moving slowly. That said, if they are at the same speed , the vehicles' weight then come into picture.

Re: A compelling optical illusion called the illusion of [#permalink]

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20 Feb 2013, 23:28

This is classic example of Assumption question where we need not put Math knowledge in the POE.

we have to assume other things being equal , optical illusion is justified for the time(car) <time(truck)

Hope that helps !!
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Re: A compelling optical illusion called the illusion of [#permalink]

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21 Feb 2013, 03:34

I prethink an assumption before going to answer choices.

my prethinking: motorist is affected by ilusion.

clearly, this prethought assumption is different from the assumption in oa. but when I read b, I immediately realize this assumption.

the prethinking process help us better understand the argument because we already know one assumption. the point is that it is easier to realize a new assumption (in oa) when we already know an assumption which we prethink.

I think this point is important.

pls comment on my thinking.
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I prethink an assumption before going to answer choices.

my prethinking: motorist is affected by ilusion.

clearly, this prethought assumption is different from the assumption in oa. but when I read b, I immediately realize this assumption.

the prethinking process help us better understand the argument because we already know one assumption. the point is that it is easier to realize a new assumption (in oa) when we already know an assumption which we prethink.

I think this point is important.

pls comment on my thinking.

Hi thangvietnam,

Yes you are right, the importance of prethinking is to understand the argument before you jump into the answer choice. Even if you do not get the correct answer choice, you have a solid understanding of the argument before you attack the answer choices.

Re: A compelling optical illusion called the illusion of [#permalink]

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13 Apr 2013, 17:39

Blue Book CR67

Strengthen Conclusion: Estimated that small car takes LESS time than large truck. Why? Premise: (On the basis of illusion of v and s) Appeared that larger objects to have less speed, thus incurred more time.

Not A: If assumed that truck's speed less than car, then estimation is not necessary since the case presents that it must always be true that car will take LESS time. It's B: If assumed that truck's speed is same as car (holding all other factors, in this case is velocity, constant), this is in accordance with the illusion theory that the estimation is based on the appearance that car takes LESS time than large truck.

Last edited by margaretgmat on 13 Apr 2013, 18:31, edited 1 time in total.

Re: A compelling optical illusion called the illusion of [#permalink]

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13 Apr 2013, 18:14

IMHO answer is B.

This question type is a find the assumption, which must be true otherwise the conclusion falls apart. The conclusion is motor estimate when looking at small car is smaller than when looking at bigger truck and this is because of optical illusion.

B is the only answer which destroys the conclusion if negated - The speed of approaching car and truck are not the same..

//kudos please, if this explanation is good.
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