求助2道GMAT的书学题

zetahisoka · 发表于 2011-2-20 21:00:44

1. Of the 60 animals on a certain farm, 2/3 are either pigs or cows. How many of the animals are cows?
<1> The farm has more than twice as many cows as it has pigs.
<2>The farm has more than 12 pigs.

我选的是A。答案是D。不明白<2>为什么是充分的。

2. For all postive integers m, m*=3m, when m is odd, m*=1/2m when m is even. Which one of the following is 9*乘6*？

答案是 27。我做出来是81。

请知道的朋友们帮忙解决下吧，谢谢~

sdcar2010 · 发表于 2011-2-20 21:55:37

of the 60 animals on a certain farm, 2/3 are either pigs or cows. how many of the animals are cows?
1) the farm has more than twice as many cows as it has pigs.
2) the farm has more than 12 pigs.

So among the 60 animals are 40 pigs and cows.

1) number of cows > 2*[numbers of pigs]. Two many possibilities. 39 cows and 1 pig; 38 cows and 2 pigs; etc. Insufficient.
2) number of pigs > 12. Again two many possibilities. 39 pigs and 1 cows; 38 pigs and 2 cows, etc. Insufficient.

1) and 2). Based on 2) At least 13 pigs. Then according to 1), at least 27 cows. But wait. 13+ 27 = 40. That's it. That's the only possibility. Sufficient.

sdcar2010 · 发表于 2011-2-20 21:58:56

For all postive integers m, [m]=3m, when m is odd, [m]=1/2m when m is even. Which one of the following is [9]乘[6]？

答案是 [27]。我做出来是[81]。

For this one, the answer is [27], not [81] if you read the answer correctly.

According to the stimulus, [9] = 3*9 =27. [6] = (1/2) * 6 = 3. So [9]*[6] = 27 * 3 = 81. So far so good. But if you choose (a) as the correct answer, you were wrong because (a) is [81] = 3*81 = 243!

On the other hand, (d) [27] = 3*27 = 81, is the right answer!

zetahisoka · 发表于 2011-2-20 23:19:08

thank you so much for help.

But the 1st Q, answer is D. coz if the farm has more than twice as many cows as it has pigs. that means there are 40 cows, 20 pigs. Sufficient.

I want to know why the sencond one is sufficent?

sdcar2010 · 发表于 2011-2-20 23:59:53

Of the 60 animals on a certain farm, 2/3 are either pigs or cows.

This means, 40 animals are either pigs or cows. Remember, animals include more than just pigs or cows.

Otherwise, the sentence should read as:

Of the 60 pigs and cows on a certain farm, 2/3 are the same type of animal.

zetahisoka · 发表于 2011-2-21 19:54:16

Thank you, I get it, and the answer is C. I missed the point that 2/3 of the animals are either pigs or cows means 40 is the total number of cows and pigs....

求助2道GMAT的书学题

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