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505-555 (Easy)|   Method of Reasoning|                     
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KarishmaB
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A compelling optical illusion called the illusion of velocity and size 28 Jul 2022, 02:00
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zoezhuyan
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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


Official Guide for GMAT Verbal Review, 2nd Edition

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

Premise:
An illusion makes objects appear to be moving more slowly the larger the objects are.

Conclusion: 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 only fact given is that larger an object, the slower it appears to be moving (than its actual speed). So a truck moving at 50 mph will appear to be slower than a car moving at 50 mph, which in turn will appear to be moving slower than a bike moving at 50 mph.

From this we conclude that estimate of time available to cross with a smaller car approaching will be lower than that with a truck approaching.
So if I have to cross the road and a car is approaching, I might think that I have 5 secs available to cross. But if a truck is approaching, I might think I have 8 secs to cross (because the truck will seem to be moving slower)
When does this conclusion make sense? When they both are moving at the same speed. Then the motorist's estimate of time available will be lower in case of the car (it will seem to him that the car is faster than the truck)

What if the truck is actually moving much faster than the car? Then his estimate of time available may be same as that for the car or may even be smaller than that for the car.
What if the truck is moving much slower than the car? Then his estimate of time available in case of car will be much lower than that in case of the truck. But then, what role does the illusion have to play in this? None. Even without the info on illusion we can conclude what we concluded.

The logic of the argument works only when the speed of the two is assumed to be the same. Answer (B)


zoezhuyan
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hi KarishmaB
and egmat, egmat

GMATNinjaTwo, VeritasKarishma, MartyTargetTestPrep, AndrewN,VeritasPrepBrian,GMATRockstar

I reviewed your points over and over, and I am still have a question.
how can you know the fact that larger an object, the slower it appears to be moving (than its actual speed), why not the fact that larger an object, the slower it appears to be moving (than smaller's speed)
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zoezhuyan - The point is that a larger object appears slower than a smaller object of the same speed.

The actual speeds of the objects are irrelevant. As I mentioned above while explaining option (E), when an object is moving at 50 mph, whether my estimate is less than that, equal to that or more than that depends on my estimating skill.
The phrase "(than its actual speed)" needed to be removed from above which I have done.

The only thing we are given is that when two objects are moving at the same speed, the larger one appears slower than the smaller one (though they both have the same speed). Keeping this in mind, review the question once more and see if it makes sense.
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A compelling optical illusion called the illusion of velocity and size 28 Jul 2022, 23:29
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zoezhuyan - The point is that a larger object appears slower than a smaller object of the same speed.

The actual speeds of the objects are irrelevant. As I mentioned above while explaining option (E), when an object is moving at 50 mph, whether my estimate is less than that, equal to that or more than that depends on my estimating skill.
The phrase "(than its actual speed)" needed to be removed from above which I have done.

The only thing we are given is that when two objects are moving at the same speed, the larger one appears slower than the smaller one (though they both have the same speed). Keeping this in mind, review the question once more and see if it makes sense.
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hi KarishmaB
I haven't read "the same speed" in the argument,i think I must missed something, but I am sure what I missed.
would you please explain further?

hope I did not disturb you much.

have a nice day.
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A compelling optical illusion called the illusion of velocity and size 28 Jul 2022, 23:49
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zoezhuyan
KarishmaB


zoezhuyan - The point is that a larger object appears slower than a smaller object of the same speed.

The actual speeds of the objects are irrelevant. As I mentioned above while explaining option (E), when an object is moving at 50 mph, whether my estimate is less than that, equal to that or more than that depends on my estimating skill.
The phrase "(than its actual speed)" needed to be removed from above which I have done.

The only thing we are given is that when two objects are moving at the same speed, the larger one appears slower than the smaller one (though they both have the same speed). Keeping this in mind, review the question once more and see if it makes sense.

hi KarishmaB
I haven't read "the same speed" in the argument,i think I must missed something, but I am sure what I missed.
would you please explain further?

hope I did not disturb you much.

have a nice day.
Show more

That is the part the argument wants us to understand and select from the options.

Look at the given data:
... the illusion of velocity and size makes objects appear to be moving more slowly the larger the objects are.

What does this mean? Say an object is moving at 50 mph. You see it moving and see that it has a certain speed. If you were to guess, you might say it is moving at 40 mph or 50 mph or 60 mph whatever.
Now say the object is double stacked and becomes larger. The "illusion" says that now it will seem to you to be moving at a slower speed than before i.e. now you may guess that it is moving at 30 mph or 40 mph or 50 mph respectively (depending on what you guessed before)

What information will make the above given meaningful? If we know that the object's speed did not change when its size was increased, then it becomes meaningful. Then we know that an illusion exists such that though the speed does not change, we feel that it has reduced because the object has become larger.
If the object actually slowed down when it became larger, then it is not an illusion anymore.

That is why the conclusion (an application of the illusion principle) can be properly drawn if it is known that the speed is the same in both cases. That is how we select option (B) as the answer.
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A compelling optical illusion called the illusion of velocity and size 20 May 2023, 02:24
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The given statement explains that the illusion of velocity and size causes objects to appear to be moving more slowly as they become larger. Based on this, it is concluded that a motorist's estimate of the time available for crossing a highway with a small car approaching will be lower than it would be with a large truck approaching. Let's evaluate each option to determine which clarifications would strengthen the conclusion:

(A) The truck's speed is assumed to be lower than the car's.
This option does not strengthen the conclusion. While it introduces a speed comparison between the car and the truck, it does not directly address or clarify the impact of the illusion of velocity and size on the motorist's estimate of time available for crossing the highway.

(B) The truck's speed is assumed to be the same as the car's.
This option strengthens the conclusion. If the truck's speed is assumed to be the same as the car's, it reinforces the notion that the illusion of velocity and size is the primary factor influencing the motorist's estimate of time available. As the larger truck approaches, it would appear to be moving more slowly due to the optical illusion, leading the motorist to perceive less time available for crossing the highway compared to a smaller car.

(C) The truck's speed is assumed to be higher than the car's.
This option weakens the conclusion. If the truck's speed is assumed to be higher than the car's, it contradicts the claim that the illusion of velocity and size makes objects appear to move more slowly as they become larger. In this scenario, the larger truck would appear to be moving faster, which would not support the conclusion that the motorist's estimate of time available is lower with a small car approaching than with a large truck approaching.

(D) The motorist's estimate of time available is assumed to be more accurate with cars approaching than with trucks approaching.
This option weakens the conclusion. If the motorist's estimate of time available is assumed to be more accurate with cars approaching than with trucks approaching, it contradicts the claim that the illusion of velocity and size influences the motorist's perception of time. The conclusion relies on the assumption that the illusion causes the motorist's estimate to be lower when a small car approaches, suggesting that the estimate is less accurate. This option suggests the opposite.

(E) The motorist's estimate of time available is assumed to be more accurate with trucks approaching than with cars approaching.
This option strengthens the conclusion. If the motorist's estimate of time available is assumed to be more accurate with trucks approaching than with cars approaching, it supports the claim that the illusion of velocity and size affects the motorist's perception of time. It reinforces the idea that the illusion makes objects, such as larger trucks, appear to move more slowly, leading the motorist to have a higher estimate of time available when a large truck approaches compared to a small car.

In conclusion, the conclusion provided in the statement would be more properly drawn if it were made clear that the truck's speed is assumed to be the same as the car's (option B). This strengthens the conclusion by reinforcing the notion that the illusion of velocity and size influences the motorist's estimate of time available, as the larger truck would appear to be moving more slowly. The other options either weaken the conclusion or introduce conflicting assumptions.
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A compelling optical illusion called the illusion of velocity and size 05 Aug 2025, 04:46
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