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Rates problem

by gmattesttaker2 » Sun Jul 29, 2012 10:13 am
Hello,

Hope all is well. I was wondering if you can please assist here?

Did it take a certain ship less than 3 hours to travel 9 kilometers ( 1 km = 1000 meters)

1) The ship's average speed over the 9 kms was greater than 55 meters per minute
2) The ship's average speed over the 9 kms was less than 60 meters per minute

Ans: A


Thanks for your valuable time and help.

Best Regards,
Sri
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by eagleeye » Sun Jul 29, 2012 12:41 pm
gmattesttaker2 wrote:Hello,

Hope all is well. I was wondering if you can please assist here?

Did it take a certain ship less than 3 hours to travel 9 kilometers ( 1 km = 1000 meters)

1) The ship's average speed over the 9 kms was greater than 55 meters per minute
2) The ship's average speed over the 9 kms was less than 60 meters per minute

Ans: A


Thanks for your valuable time and help.

Best Regards,
Sri
We need to find whether 9km less than 3 hours. Since the options are in meters per minute. Let's rephrase the question. speed to complete the journey in 3 hours = 180 minutes = 9000m/(180min) = 50 meters/minute.
So if the speed is greater than 50 meters per minute, the journey will be completed in less than 3 hours, otherwise it won't be. So we need to check is speed > 50 meters per minute.

With that in mind, let's look at the options:

Did it take a certain ship less than 3 hours to travel 9 kilometers ( 1 km = 1000 meters)

1) The ship's average speed over the 9 kms was greater than 55 meters per minute
Clearly, speed is greater than 50. Sufficient.

2) The ship's average speed over the 9 kms was less than 60 meters per minute
We don't know whether speed is greater than 50 or less than 50. Insufficient.

Hence, A is correct.
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by gmattesttaker2 » Sun Jul 29, 2012 7:41 pm
eagleeye wrote:
gmattesttaker2 wrote:Hello,

Hope all is well. I was wondering if you can please assist here?

Did it take a certain ship less than 3 hours to travel 9 kilometers ( 1 km = 1000 meters)

1) The ship's average speed over the 9 kms was greater than 55 meters per minute
2) The ship's average speed over the 9 kms was less than 60 meters per minute

Ans: A


Thanks for your valuable time and help.

Best Regards,
Sri
We need to find whether 9km less than 3 hours. Since the options are in meters per minute. Let's rephrase the question. speed to complete the journey in 3 hours = 180 minutes = 9000m/(180min) = 50 meters/minute.
So if the speed is greater than 50 meters per minute, the journey will be completed in less than 3 hours, otherwise it won't be. So we need to check is speed > 50 meters per minute.

With that in mind, let's look at the options:

Did it take a certain ship less than 3 hours to travel 9 kilometers ( 1 km = 1000 meters)

1) The ship's average speed over the 9 kms was greater than 55 meters per minute
Clearly, speed is greater than 50. Sufficient.

2) The ship's average speed over the 9 kms was less than 60 meters per minute
We don't know whether speed is greater than 50 or less than 50. Insufficient.

Hence, A is correct.

Hello Eagleeye,

Many thanks for your awesome explanation (as always!).

Best Regards,
Sri
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by tabsang » Sun Sep 01, 2013 6:12 am
Hi,

Doesn't the word "Average Speed" throw this off?

Shouldn't the answer then be [spoiler](E)[/spoiler]?

It's quite possible that say for the first 3.01 hours the ship travels abysmally slowly and then picks up crazy speed such that the average speed is more than 55?

Just a thought. Experts can you help. Pretty Please :) :)

Cheers,
Taz
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by Brent@GMATPrepNow » Sun Sep 01, 2013 6:31 am
tabsang wrote:Hi,

Doesn't the word "Average Speed" throw this off?

Shouldn't the answer then be [spoiler](E)[/spoiler]?

It's quite possible that say for the first 3.01 hours the ship travels abysmally slowly and then picks up crazy speed such that the average speed is more than 55?

Just a thought. Experts can you help. Pretty Please :) :)

Cheers,
Taz
Hey Taz,

Good question.

Once we know the ship's average speed over the entire 9 km, then we know EXACTLY how long it took the ship to complete the 9 km distance. So, it doesn't matter whether or not the ship traveled at a constant speed.

Cheers,
Brent
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by Brent@GMATPrepNow » Sun Sep 01, 2013 6:49 am
gmattesttaker2 wrote:Hello,

Hope all is well. I was wondering if you can please assist here?

Did it take a certain ship less than 3 hours to travel 9 kilometers ( 1 km = 1000 meters)

1) The ship's average speed over the 9 kms was greater than 55 meters per minute
2) The ship's average speed over the 9 kms was less than 60 meters per minute
I love eagleeye's method of rephrasing the target question before examining the two statements. In fact, I think it's the best approach here. Aside: We have a free video with tips on rephrasing the target question: https://www.gmatprepnow.com/module/gmat- ... cy?id=1100

That said, we can still solve this without rephrasing the target question.

Target question: Was the ship's travel time less than 3 hours?

Statement 1: The ship's average speed over the 9 kms was GREATER THAN 55 meters per minute
Let's determine the travel time if the average speed were EXACTLY 55 meters per minute.
First, we'll find the equivalent speed in KM per HOUR
55 meters per minute = 55(60) meters per hour = 3300 meters per hour = 3.3 KM per HOUR
time = distance/speed
So, time = 9km/3.3kmh = less than 3 hours
So, when the ship's average speed is EXACTLY 55 meters per minute, the travel time is less than 3 hours.
So, if the ship's average speed is GREATER THAN 55 meters per minute, the ship's travel time is definitely less than 3 hours
Since we can answer the target question with certainty, statement 1 is SUFFICIENT

Statement 2: The ship's average speed over the 9 kms was LESS THAN 60 meters per minute
Let's consider two possible cases:
case a: the ship's average speed was 55 meters per minute. We already learned, from statement 1, that an average speed of 55 meters per minute means the ship's travel time is less than 3 hours.
case b: the ship's average speed was 0.001 meters per minute. If we choose such a crazy slow speed, we shouldn't need to perform any calculations. It should be obvious that, at that speed, the ship's travel time is greater than 3 hours.
Since we cannot answer the target question with certainty, statement 2 is NOT SUFFICIENT

Answer = A

If anyone is interested, here's similar question to practice with: https://www.beatthegmat.com/carlos-the-r ... 64859.html

Cheers,
Brent
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by tabsang » Sun Sep 01, 2013 7:54 am
Brent@GMATPrepNow wrote:
tabsang wrote:Hi,

Doesn't the word "Average Speed" throw this off?

Shouldn't the answer then be [spoiler](E)[/spoiler]?

It's quite possible that say for the first 3.01 hours the ship travels abysmally slowly and then picks up crazy speed such that the average speed is more than 55?

Just a thought. Experts can you help. Pretty Please :) :)

Cheers,
Taz
Hey Taz,

Good question.

Once we know the ship's average speed over the entire 9 km, then we know EXACTLY how long it took the ship to complete the 9 km distance. So, it doesn't matter whether or not the ship traveled at a constant speed.

Cheers,
Brent
Thanks Brent :)
*Awesome as always*

That helps.

Cheers,
Taz
If this post helped you in your GMAT prep, please take a moment and hit the "thanks" button. It'll make my day :) :)
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