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Intern  Joined: 23 Nov 2010
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What is the value of x?  [#permalink]

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Question Stats: 26% (01:43) correct 74% (01:28) wrong based on 490 sessions

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What is the value of x?

(1) x^3 is a 2-digit positive odd integer.
(2) x^4 is a 2-digit positive odd integer.
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shan123 wrote:
What is the value of x?
(1) X3 is a 2-digit positive odd integer. (2) X4 is a 2-digit positive odd integer.

I don't know whether the answer is correct. I got a different one.

What is the value of x?

Note that we are not told that x is an integer

(1) x^3 is a 2-digit positive odd integer --> now, if $$x$$ is an integer then $$x=3$$ as $$x^3=27$$ is the only odd 2-digit positive cube of an integer (1^3=1 and 5^3=125) but if $$x$$ is not an integer then it can be cube root of any 2-digit positive odd integer, for example if $$x=\sqrt{11}$$ then $$x^3=11$$. Not sufficient.

(2) x^4 is a 2-digit positive odd integer --> basically the same here: if $$x$$ is an integer then $$x=3$$ or $$x=-3$$ as $$x^4=81$$ is the only odd 2-digit positive integer which is in fourth power of an integer (1^4=1 and 5^4=625) (so even if $$x$$ is an integer this statement is still insufficient as it gives two values for $$x$$: 3 and -3). $$x$$ also can be non-integer as above: it can be fourth root from any 2-digit positive odd integer, for example if $$x=\sqrt{11}$$ then $$x^4=11$$. Not sufficient.

(1)+(2) $$x$$ cannot be an irrational number (so that both x^3 and x^4 to be integers), so $$x$$ must be 3. Sufficient.

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Bunuel wrote:
shan123 wrote:
What is the value of x?
(1) X3 is a 2-digit positive odd integer. (2) X4 is a 2-digit positive odd integer.

I don't know whether the answer is correct. I got a different one.

What is the value of x?

Note that we are not told that x is an integer

(1) x^3 is a 2-digit positive odd integer --> now, if $$x$$ is an integer then $$x=3$$ as $$x^3=27$$ is the only odd 2-digit positive cube of an integer (1^3=1 and 5^3=125) but if $$x$$ is not an integer then it can be cube root of any 2-digit positive odd integer, for example if $$x=\sqrt{11}$$ then $$x^3=11$$. Not sufficient.

(2) x^4 is a 2-digit positive odd integer --> basically the same here: if $$x$$ is an integer then $$x=3$$ or $$x=-3$$ as $$x^4=81$$ is the only odd 2-digit positive integer which is in fourth power of an integer (1^4=1 and 5^4=625) (so even if $$x$$ is an integer this statement is still insufficient as it gives two values for $$x$$: 3 and -3). $$x$$ also can be non-integer as above: it can be fourth root from any 2-digit positive odd integer, for example if $$x=\sqrt{11}$$ then $$x^4=11$$. Not sufficient.

(1)+(2) $$x$$ can not be an irrational number (so that both x^3 and x^4 to be integers), so $$x$$ must be 3. Sufficient.

Thanks for the answer and detailed explanation.
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Carelessly, I overlooked the possibility that x could be negative. Thanks Bunuel!
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Tricky one, I considered the integer constraint that didn't exist. Must take care with this.

Bunuel wrote:
shan123 wrote:
What is the value of x?
(1) X3 is a 2-digit positive odd integer. (2) X4 is a 2-digit positive odd integer.

I don't know whether the answer is correct. I got a different one.

What is the value of x?

Note that we are not told that x is an integer

(1) x^3 is a 2-digit positive odd integer --> now, if $$x$$ is an integer then $$x=3$$ as $$x^3=27$$ is the only odd 2-digit positive cube of an integer (1^3=1 and 5^3=125) but if $$x$$ is not an integer then it can be cube root of any 2-digit positive odd integer, for example if $$x=\sqrt{11}$$ then $$x^3=11$$. Not sufficient.

(2) x^4 is a 2-digit positive odd integer --> basically the same here: if $$x$$ is an integer then $$x=3$$ or $$x=-3$$ as $$x^4=81$$ is the only odd 2-digit positive integer which is in fourth power of an integer (1^4=1 and 5^4=625) (so even if $$x$$ is an integer this statement is still insufficient as it gives two values for $$x$$: 3 and -3). $$x$$ also can be non-integer as above: it can be fourth root from any 2-digit positive odd integer, for example if $$x=\sqrt{11}$$ then $$x^4=11$$. Not sufficient.

(1)+(2) $$x$$ can not be an irrational number (so that both x^3 and x^4 to be integers), so $$x$$ must be 3. Sufficient.

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Re: What is the value of x?  [#permalink]

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carcass wrote:
What is the value of$$x$$ ?

(1) $$X^3$$ is a 2-digit positive odd integer.

(2)$$X^4$$ is a 2-digit positive odd integer.

Hi carcass,

Stat 1 :

Only 2 digit positive integers for S1 are :
$$x$$-------- 3 ------ 4
$$x^3$$ ----27-----64

Here odd integer is x=3 and x^3 = 27
SUFFICIENT

Stat 2 :

Only 2 digit positive integers for S2 are :
$$x$$----------+/-2-------------+/-3
$$x^3$$----------16----------81

Here odd integer is x=+/-3 and x^3 = 81
INSUFFICIENT (two values for x)

IMO A.

But how come C?
did i missed out anything?
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Re: What is the value of x?  [#permalink]

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Shanmugam, the problem doesnt explicitly state that x is an integer. It can be fraction.

e.g. Choice (A), x can be fraction -> $$x^3 = 35$$ i.e. x = $$\sqrt{35}$$

Similarly Choice (B) alone is not sufficient.

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Finance your Student loan through SoFi and get $100 referral bonus : Click here Senior Manager  Joined: 27 Jun 2012 Posts: 347 Concentration: Strategy, Finance Schools: Haas EWMBA '17 Re: What is the value of x? [#permalink] ### Show Tags 3 Glad that helped. Always watch out for ZIP trap (assuming Zero, Integer, Positive) -> (Make sure to check for 0, factions and negatives) Especially for inequalities, algebraic, number/fraction problems. _________________ Thanks, Prashant Ponde Tough 700+ Level RCs: Passage1 | Passage2 | Passage3 | Passage4 | Passage5 | Passage6 | Passage7 Reading Comprehension notes: Click here VOTE GMAT Practice Tests: Vote Here PowerScore CR Bible - Official Guide 13 Questions Set Mapped: Click here Finance your Student loan through SoFi and get$100 referral bonus : Click here
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Re: What is the value of x?  [#permalink]

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Sorry Bunuel I do not "visualize" why in C $$x^3$$ and $$x^4$$cannot be rational numbers aka integers

because an irrational can't be at the same time an 2 digits odd number ?' and of course only 3 meets both conditions ?'

Can you explain me please ?'

Thanks
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Re: What is the value of x?  [#permalink]

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carcass wrote:
Sorry Bunuel I do not "visualize" why in C $$x^3$$ and $$x^4$$cannot be rational numbers aka integers

because an irrational can't be at the same time an 2 digits odd number ?' and of course only 3 meets both conditions ?'

Can you explain me please ?'

Thanks

Not sure I understand what you mean.

Anyway, rational numbers and integers are not the same. Also, irrational numbers are not integers, thus they can be neither odd nor even.

For more check here: math-number-theory-88376.html
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Re: What is the value of x?  [#permalink]

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basically 1) is insuff because we have to consider integers and non integers (so irrational numbers). Same for 2)

Bothe statements are suff because we have only 3 that mettes the criteria so we have to consider only the 3 (the integer). So sufficient

But why we C is sufficient ?' why we can not consider the irrational numbers ??

Thanks. Now I hope is more clear what I mean. I'm sorry if I have explained myself badly
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Re: What is the value of x?  [#permalink]

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carcass wrote:
basically 1) is insuff because we have to consider integers and non integers (so irrational numbers). Same for 2)

Bothe statements are suff because we have only 3 that mettes the criteria so we have to consider only the 3 (the integer). So sufficient

But why we C is sufficient ?' why we can not consider the irrational numbers ??

Thanks. Now I hope is more clear what I mean. I'm sorry if I have explained myself badly

If x is an irrational number then x^3 and x^4 cannot both be integers as given in the statements, so x can only be 3.
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Bunuel wrote:
shan123 wrote:

(1)+(2) $$x$$ cannot be an irrational number (so that both x^3 and x^4 to be integers), so $$x$$ must be 3. Sufficient.

Hi Bunuel: Just like others, I also have a hard time visualizing that there does not exist an irrational number whose 3rd and 4th power both result in an odd digit integer. I mean integer is a smaller set compared to irrational numbers and we still have 3 (an integer) whose 3rd and 4th power both result in an odd 2-digit integer. On the other hand in terms of irrational numbers we have tremendous possibilities even between two integers we have infinite irrational numbers and we cannot have such a number. It some how feels odd to me. I have no doubt what you are saying is right but I have hard time imagining it. Maybe my understanding of irrational numbers and their powers is still primordial.
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MensaNumber wrote:
Bunuel wrote:
shan123 wrote:

(1)+(2) $$x$$ cannot be an irrational number (so that both x^3 and x^4 to be integers), so $$x$$ must be 3. Sufficient.

Hi Bunuel: Just like others, I also have a hard time visualizing that there does not exist an irrational number whose 3rd and 4th power both result in an odd digit integer. I mean integer is a smaller set compared to irrational numbers and we still have 3 (an integer) whose 3rd and 4th power both result in an odd 2-digit integer. On the other hand in terms of irrational numbers we have tremendous possibilities even between two integers we have infinite irrational numbers and we cannot have such a number. It some how feels odd to me. I have no doubt what you are saying is right but I have hard time imagining it. Maybe my understanding of irrational numbers and their powers is still primordial.

Say x IS an irrational number and x*x*x=x^3=integer. In this case x*x*x*x=x^3*x=integer*irrational=irrational.

If x is an irrational number and x*x*x*x=x^4=integer, then x^3=x^4/x=integer/irrational=irrational.

So, as you can see if x is an irrational number, then both x^3 and x^4 cannot be rational.

Does this make sense?
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Bunuel wrote:
Say x IS an irrational number and x*x*x=x^3=integer. In this case x*x*x*x=x^3*x=integer*irrational=irrational.

If x is an irrational number and x*x*x*x=x^4=integer, then x^3=x^4/x=integer/irrational=irrational.

So, as you can see if x is an irrational number, then both x^3 and x^4 cannot be rational.

Does this make sense?

Wow! Makes complete sense. This explanation is superb. Thanks!
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Re: What is the value of x?  [#permalink]

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Innocuous looking deadly snake this one !!!
bowled me ....
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Re: What is the value of x?  [#permalink]

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since x can be any number than an integer, and x can have negative value, combine 2 statement will yield the answer.
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