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GMAT PREP 2006 #1

by Perminology » Sun Dec 20, 2009 6:14 pm
Hello all, was having some trouble with the following questions. Since GMAT Prep doesn't provide explanations, what better place to ask than at beatthegmat? here goes.

9. Did it take Pei more than 2 hours to walk a distance of 10 miles along a certain trail? (1 mile= 1.6 km)
1) Pei walked this distance at an average rate of less than 6.4 km/h
2) On average, it took Pei more than 9 minutes per km to walk this distance.

OA D Still don't understand this one.

36. The attendees of a certain convention purchased a total of 15,000 books. How many of the attendees were female?

1) There was a total of 4000 attendees at the convention
2) Males purchased an average arithmetic mean of 3 books each, and the females purchased an average of 5 books each.


OA C
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Source: — Data Sufficiency |
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by viju9162 » Sun Dec 20, 2009 8:35 pm
Hi Perminology,

My reasoning is as follows:

First sum

Question : Did pei take more than 2 hours to walk a distance of 10 miles ( Answer type should be Yes or NO)

(1) Walks at an average rate of less than 6.4 km/hr

Assume he walks with the speed 6.4 km/hr( This is the maximum).

Therefore, 6.4 = 10 * 1.6 / time.
=> time = 2.5 hours.

With his maximum speed, pei has taken 2.5 hours. Even if you gradually reduce his speed, he will take more time to travel. Hence, A is sufficient.

(2) On average, pei takes more than 9 minutes/km

Pei takes for example 10 minutes ( minimum) ( because it is more than 9 mins), to travel 1 km. Therefore, by proportion.

For 1 km, he takes 10/60 hour, .i.e 1/6 hour to travel, then
For 10*1.6 km, how much time will he take?

He will take at least 2.6 hours to cover the distance. As he takes more time to travel 1 km, he will definitely take more than 2.6 hours to cover the whole distance.

Therefore, B is also sufficient.

From the above (1) and (2), answer should be
D

Second Sum

Question: How many are females?

From the question, we know that 15,000 books are sold ( keep this info, we will use it in the solution)

(1) Total of 4,000 people attended.

If n(m) - number of males and n(f) - number of females.
Then n(m) + n(f) = 4,000

Cannot find number of females

(2) average is mentioned for both male and female

3 = T(M) / n(m)
5 = T(F) /n(f)

T(m) - Total books purchased by males
T(F) - Total books purcahsed by females

Cannot find about number of females

Combine both (1) and (2)

From the question, we know T(M) + T(F) = 15,000 . Substitute T(M) and T(F) from(2)

3*n(m) + 5*n(f) = 15,000
from ( 1) n(m) = 4,000 - n(f)
3* ( 4,000 - n(f)) + 5*n(f) = 15,000
n(f) = 1500

You got the answer :-).

Regards,
Viju
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by Perminology » Sun Dec 20, 2009 11:07 pm
Hey Viju, thanks for the great post. The ratio question seems so easy now. In fact I've done questions like that before and got them right. Careless mistakes tend to be my biggest enemy on the GMAT. I appreciate your input for each question, but I have big beef with the Rate Time Distance questions. Tell me if you think what I have to say has some weight.

1) Walks at an average rate of less than 6.4 km/hr

We shouldn't assume that he did walk at 6.4, for all we know it could be 2.1, or 0.1. Thus we would have multiple values for time. For the same reason x<0, x could be a fraction or negative, there's no way of knowing for sure without sufficient information. Thus I thought statement 1 was insufficient.

2) On average, pei takes more than 9 minutes/km

Same problem here. We're given a minimum. Ignoring statement 1, he could have walked 120 minutes/km. The range of values is so large that it is insufficient. It's like asking "what is the value of x?" with a choice x>15. Insufficient right?

Thus, combining both we're given a range of rates. Thus if we can calculate for the range of rates, then we can know min/max of time based on two definite limitations for rates, while each alone is so ambiguous. So I chose C. Thoughts?
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by viju9162 » Sun Dec 20, 2009 11:26 pm
Hi Perminology,

You are right to certain extent, but you didn't consider other values given in the question. The author states that pei walks at an average less than 6.4 km/hr;however, the author also gives you the distance. This information needs to be utilized to arrive at the answer. I think you missed here.

Similarly, for choice 2, you relied on only one data. It is good to utilize the given data to arrive at the answer. The information is spread across in question, answer, ...

I hope this helps.

Regards,
Viju
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by Perminology » Sun Dec 20, 2009 11:47 pm
Thanks Viju, looks like I obsessed over the range. The fact that the distance is supplied definitely makes each choice sufficient. Thanks for your help, it was the first time I encountered a rate time distance question with inequalities.
Thanks!
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