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Hi Bunuel,
what confused me in the solution is that if -x^2 and -x^3 are -1/4 and -1/8, then how are we again treating them as x individually. The way question is written it seems that the set has 3 numbers, x, -x^2, and -x^3

Please help me out with this, thanks :)
Bunuel
5. For a certain set of numbers, if \(x\) is in the set, then both \(-x^2\) and \(-x^3\) are also in the set. If the number \(\frac{1}{2}\) is in the set, which of the following must also be in the set?

I. \(-\frac{1}{64}\)

II. \(\frac{1}{64}\)

III. \(\frac{1}{\sqrt[3]{2}}\)


A. I only,
B. II only,
C. III only,
D. I and II only
E. I, II and III

Since \(\frac{1}{2}\) is in the set, the following must also be in the set:


\(-x^2 = -\frac{1}{4}\);

\(-x^3 = -\frac{1}{8}\).

Since \(-\frac{1}{4}\) is in the set, the following must also be in the set:


\(-x^3 =\frac{1}{64}\)

Since \(-\frac{1}{8}\) is in the set, the following must also be in the set:


\(-x^2 =-\frac{1}{64}\)

The only number we cannot obtain is \(\frac{1}{\sqrt[3]{2}}\).

Answer: D.
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saransh2797
Hi Bunuel,
what confused me in the solution is that if -x^2 and -x^3 are -1/4 and -1/8, then how are we again treating them as x individually. The way question is written it seems that the set has 3 numbers, x, -x^2, and -x^3

Please help me out with this, thanks :)
Bunuel
5. For a certain set of numbers, if \(x\) is in the set, then both \(-x^2\) and \(-x^3\) are also in the set. If the number \(\frac{1}{2}\) is in the set, which of the following must also be in the set?

I. \(-\frac{1}{64}\)

II. \(\frac{1}{64}\)

III. \(\frac{1}{\sqrt[3]{2}}\)


A. I only,
B. II only,
C. III only,
D. I and II only
E. I, II and III

Since \(\frac{1}{2}\) is in the set, the following must also be in the set:


\(-x^2 = -\frac{1}{4}\);

\(-x^3 = -\frac{1}{8}\).

Since \(-\frac{1}{4}\) is in the set, the following must also be in the set:


\(-x^3 =\frac{1}{64}\)

Since \(-\frac{1}{8}\) is in the set, the following must also be in the set:


\(-x^2 =-\frac{1}{64}\)

The only number we cannot obtain is \(\frac{1}{\sqrt[3]{2}}\).

Answer: D.

The set's rule means each number generates new elements based on the pattern. Here, -x^2 and -x^3 are treated as elements in the set, not individual variables. They follow the same rule to generate further numbers.
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11. Certain bowl contains 5 red marbles and 3 blue marbles only. One by one, every marble is drawn at random and without replacement. What is the probability that the seventh marble drawn is NOT blue?

A. 7/8
B. 3/4
C. 2/3
D. 5/8
E. 3/8

Total arrangements possible: 8!/(5!*3!) = 56

Arrangements with Blue at 7th: 7!(5!*2!) = 21 (Position for 1 Blue Marble is fixed, arrange the remaining marbles)
Arrangements with Blue NOT at 7th: 56-21 = 35

Probability: 35/56 = 5/8

Answer: D

Is this a good approach? Or was it a fluke that I got it right with this method??
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But by the 7th marble, we only have 2 marbles to pick from because it is without replacement. If there's only two left, it means atleast one of the blue marbles have already been picked. Now, the the remaining two could be both blue, 1 blue or no blue. How can we say 5/8? We have to do each cases individually, subtract and then add right?
Bunuel
11. Certain bowl contains 5 red marbles and 3 blue marbles only. One by one, every marble is drawn at random and without replacement. What is the probability that the seventh marble drawn is NOT blue?
A. 7/8
B. 3/4
C. 2/3
D. 5/8
E. 3/8

Basically, we need to find the probability that the seventh marble drawn is red (so, not blue).

Now, the initial probability of drawing a red marble is \(\frac{5}{8}\). Without knowing the other results, the probability of drawing a red marble will not change for ANY successive draw: the second, third, fourth, ..., and seventh. Thus, the probability that the seventh marble is red is \(\frac{5}{8}\).

The same applies to the blue marble: the probability of drawing a blue marble is \(\frac{3}{8}\). The probability that, for instance, the 8th marble drawn is blue is still \(\frac{3}{8}\). There is simply no reason to believe WHY any draw would be different from another (provided we don't know the other results).

Answer: D.
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Sreeveluri
But by the 7th marble, we only have 2 marbles to pick from because it is without replacement. If there's only two left, it means atleast one of the blue marbles have already been picked. Now, the the remaining two could be both blue, 1 blue or no blue. How can we say 5/8? We have to do each cases individually, subtract and then add right?

By the 7th draw, the remaining marbles do depend on what happened in the first 6 draws, but we are not given any information about those draws.

Before the drawing starts, the 8 marbles are simply being placed in a random order. Any one of the 8 marbles is equally likely to be in the 7th position, and since 5 of the 8 marbles are red, the probability that the 7th marble is not blue is 5/8.

Consider this: there are 8 cards, 5 spades, and 3 hearts. What is the probability that the first card you pick will be a spade? Clearly 5/8.

Now, the next question: suppose I throw away three cards at random, not telling you which cards I have thrown away. What is the probability NOW that you pick a spade out of the 5 remaining cards? I've just reduced the sample from which you pick, but does the probability change? WHY should it change?

Another question, using the same cards (5 spades and 3 hearts): I say that you can pick one card, but only out of randomly selected 5 cards. What is the probability that the card you pick will be a spade?

There is no difference in ALL the above cases, and the probability will remain 5/8. If it changed, increased (or decreased), it would mean that the probability of picking a heart, on the other hand, decreased (or increased). Why would it?

Similar questions to practice:
https://gmatclub.com/forum/a-box-contai ... 90272.html
https://gmatclub.com/forum/a-bag-contai ... 00023.html
https://gmatclub.com/forum/each-of-four ... 01553.html
https://gmatclub.com/forum/if-40-people ... 97015.html
https://gmatclub.com/forum/a-bag-contai ... 00023.html

Hope this helps.
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