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Math Expert V
Joined: 02 Sep 2009
Posts: 58954
Re: If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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russ9 wrote:
Bunuel wrote:
russ9 wrote:

You should use whichever approach suits you the best and gives the correct answer in minimum time.

As for my solution, I divided by 3 because this way I directly get the range for x/3 ($$1<\frac{x}{3}<33\frac{1}{3}$$), and it becomes easier to evaluate the number of values for it.

Hi Bunuel,

Can you perhaps recommend a few similar problems?

Thanks!

Check this one: for-how-many-values-of-k-is-12-12-the-least-common-multiple-86737.html
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Manager  Joined: 07 Apr 2014
Posts: 99
Re: If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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1
If 3 < x < 100, for how many values of x is x/3 the square of a prime number?

(A) Two
(B) Three
(C) Four
(D) Five
(E) Nine

x/3 the square of a prime number ; 3 < x < 100..

divide the above by 3 then 1 < x/3 < 33..

so x/3 could be 4, 9, 16 25 .. since 16 is a square of 4 , which is not a prime answer should be 3.
Intern  Joined: 16 Feb 2015
Posts: 4
Re: If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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I got B

x/3 = (prime)^2
x = 3(prime) ^2

The only values for x that are 3 < x < 100 are if the prime numbers are 2,3 and 5. Once you hit 7 your x value becomes greater than 100. Therefore there are only 3 values for x.
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Location: India
Concentration: Strategy, General Management
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Re: If 3 < x < 100, for how many values of x is the square of a prime numb  [#permalink]

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X < 100

So, square root of x has to be less than 10.

Also, square is positive, square root has to be positive (as prime numbers are never negative).

The following numbers between 1 and 10 are prime: 2, 3, 5, 7.

Their squares will lie between 4 and 100.

So, (C).
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SVP  B
Joined: 06 Nov 2014
Posts: 1871
Re: If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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nahid78 wrote:
If 3 < x < 100, for how many values of x is the square of a prime number?

(A) Two
(B) Three
(C) Four
(D) Five
(E) Nine

Hi nahid78,

The values would be 4, 9, 25, 49, 81

But if you have mistyped the question and we are required to find the values of (x/3)
then the values would be 12, 27, 75. Three values
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Re: If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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If 3 < x < 100, for how many values of x is x/3 the square of a prime number?

(A) Two
(B) Three
(C) Four
(D) Five
(E) Nine

Assume x=+int
mistake mine: (x/3)^2=P wrong
(x/3) = P^2
Director  P
Joined: 04 Sep 2015
Posts: 663
Location: India
WE: Information Technology (Computer Software)
If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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If 3 < x < 100, for how many values of x is x/3 the square of a prime number?

(A) Two
(B) Three
(C) Four
(D) Five
(E) Nine

The question confusingly adds x/3... lol so the square is the value of x/3

so we take first prime numbers 2,3,5,7,11 and are 4,9,25,49,121..

Since these values should be x/3 and the value of x will be 4*3,9*3,25*3,49*3..... which are 12,27,75,147....

we only need values below 100 so we have only 12,27 and 75 Three is the answer.B
Intern  B
Joined: 09 Dec 2014
Posts: 5
WE: Information Technology (Health Care)
Re: If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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1
Bunuel wrote:
If 3 < x < 100, for how many values of x is x/3 the square of a prime number?

(A) Two
(B) Three
(C) Four
(D) Five
(E) Nine

Since $$3 < x < 100$$, then $$1<\frac{x}{3}<33\frac{1}{3}$$ (just divide all parts of the inequality by 3).

$$\frac{x}{3}$$ should be the square of a prime number, thus $$\frac{x}{3}$$ could be 2^2=4, 3^2=9, or 5^2=25.

How do you know to divide the inequality by 3. is there a general strategy for this?
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Joined: 07 Dec 2014
Posts: 1221
If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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If 3 < x < 100, for how many values of x is x/3 the square of a prime number?

(A) Two
(B) Three
(C) Four
(D) Five
(E) Nine

let x/3=y^2
3y^2<100
y^2<34
y^2 can only=4, 9, or 25
3
B
GMAT Club Legend  V
Joined: 12 Sep 2015
Posts: 4055
Re: If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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Top Contributor
1
If 3 < x < 100, for how many values of x is x/3 the square of a prime number?

(A) Two
(B) Three
(C) Four
(D) Five
(E) Nine

We want values of x (where 3 < x < 100) such that x/3 is the square of a prime number.
So, let's start checking squares of prime numbers.
Some prime numbers are 2, 3, 5, 7, 11, etc

2² = 4 and (3)(4) = 12. So, x = 12 meets the given conditions.
3² = 9 and (3)(9) = 27. So, x = 27 meets the given condition
5² = 25 and (3)(25) = 75. So, x = 75 meets the given conditions.
7² = 49 and (3)(49) = 147. No good. We need values of x such that 3 < x < 100

So, there are exactly 3 values of x that meet the given conditions.

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Intern  B
Joined: 14 May 2016
Posts: 17
Re: If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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I did an excel spreadsheet to understand this. However, the explanation from other people to divide 100/3 = 33.33... helps to reduce the amount of work.

thank you all!
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VP  D
Joined: 09 Mar 2016
Posts: 1229
Re: If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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If 3 < x < 100, for how many values of x is x/3 the square of a prime number?

(A) Two
(B) Three
(C) Four
(D) Five
(E) Nine

Hello niks18

one question: "square of a prime number" is it the same as "square number" ? just dont get the meaning of this wording "square of a prime number"

for example 9 is square number because 3*3 =9

thanks Intern  B
Joined: 15 Sep 2018
Posts: 31
Re: If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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First, we need to find the range of values for $$\frac{x}{3}$$.

Given that $$3<x<100,$$ we can divide all sides of the inequality and have

$$\frac{3}{3}<\frac{x}{3}<\frac{100}{3}$$
$$1<\frac{x}{3}<33\frac{1}{3}$$

With this range, there are only a few numbers that are squares of a prime number. They are:
$$2^2=4$$, $$3^2=9$$, $$5^2=25$$

Manager  B
Joined: 19 Jan 2018
Posts: 78
If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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If 3 < x < 100, for how many values of x is x/3 the square of a prime number?

(A) Two
(B) Three
(C) Four
(D) Five
(E) Nine

Translation:
How many values of x is x/3 the square of a prime number

$$\frac{X}{3}$$=$$p^2$$, p being a prime number

Rearrange the formula so that it equals to x:

$$X = p^2 *3$$

Now test prime numbers out:

$$X = 1^2 *3$$ X = 3 (Discard p=1 because X has to be larger than 3, not equal to it)

$$X = 2^2 *3$$ X = 12

$$X = 3^2 *3$$ X = 27

$$X = 5^2 *3$$ X = 75

$$X = 7^2 *3$$ X = 149 (Discard p=7 because x>100)

In total 3 Primes fit the condition:

Senior Manager  P
Status: Gathering chakra
Joined: 05 Feb 2018
Posts: 440
Re: If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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Have to read more carefully... wasted almost a minute thinking the question said the the prime number must be >3 rather than x (which would mean only 5 fits).

An easy way to do this is to revert the equation... x = prime#² * 3
Since x < 100, we can just test
2²*3 = 12
3²*3 = 27
5²*3 = 75
7²* 3 = 147, too big.

Manager  B
Joined: 25 Sep 2018
Posts: 63
Re: If 3 < x < 100, for how many values of x is x/3 the square  [#permalink]

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If 3 < x < 100, for how many values of x is x/3 the square of a prime number?

(A) Two
(B) Three
(C) Four
(D) Five
(E) Nine

Here,
x/3=p^2
=>x=3p^2

3*2^2=12 possible
3*3^2=27 possible
3*5^2=75 possible
But,3*7^2=147 not possible

So, x has 3 possible values

Posted from my mobile device Re: If 3 < x < 100, for how many values of x is x/3 the square   [#permalink] 11 May 2019, 05:20

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