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# In the equation 3r^{4}=(2/s), if r is doubled, then s must be

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Intern
Joined: 30 Mar 2015
Posts: 14
GMAT 1: 730 Q49 V40
In the equation 3r^{4}=(2/s), if r is doubled, then s must be  [#permalink]

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Updated on: 14 Jun 2015, 08:35
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10
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Difficulty:

45% (medium)

Question Stats:

67% (01:22) correct 33% (01:33) wrong based on 284 sessions

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In the equation $$3r^{4}$$=$$\frac{2}{s}$$, if r is doubled, the s must be multiplied by:

A.$$\frac{1}{16}$$

B. $$\frac{1}{8}$$

C.$$\frac{1}{4}$$

D. 4

E. 16

Originally posted by petu on 14 Jun 2015, 08:26.
Last edited by petu on 14 Jun 2015, 08:35, edited 1 time in total.
Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 9446
Location: Pune, India
Re: In the equation 3r^{4}=(2/s), if r is doubled, then s must be  [#permalink]

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15 Jun 2015, 23:22
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4
irus wrote:
why 1/16 can someone explain pls

$$3*r^4 = \frac{2}{s}$$
$$s= \frac{2}{3*r^4}$$ (multiply by s and divide by 3r^4)

What happens when r becomes 2r.

You get $$\frac{2}{3(2*r)^4} = \frac{2}{3*16*r^4}$$

How is this new expression different from the old one? The new one = Old one * (1/16)

Hence, s gets multiplied by 1/16 when r becomes twice.
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Karishma
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Joined: 02 Sep 2009
Posts: 56257
Re: In the equation 3r^{4}=(2/s), if r is doubled, then s must be  [#permalink]

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14 Jun 2015, 11:34
petu wrote:
In the equation $$3r^{4}$$=$$\frac{2}{s}$$, if r is doubled, the s must be multiplied by:

A.$$\frac{1}{16}$$

B. $$\frac{1}{8}$$

C.$$\frac{1}{4}$$

D. 4

E. 16

Similar question to practice: if-the-function-q-is-defined-by-the-formula-q-5w-4x-z-131051.html
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Re: In the equation 3r^{4}=(2/s), if r is doubled, then s must be  [#permalink]

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01 Nov 2015, 21:36
1
just plug in a number for r , like 1

Then solve for s , once you have s, all you have to do is double the value of r (which will now be: 2) , then solve for the new value of y
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Joined: 27 May 2012
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Re: In the equation 3r^{4}=(2/s), if r is doubled, then s must be  [#permalink]

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23 Feb 2018, 03:47
VeritasPrepKarishma wrote:
irus wrote:
why 1/16 can someone explain pls

$$3*r^4 = \frac{2}{s}$$
$$s= \frac{2}{3*r^4}$$ (multiply by s and divide by 3r^4)

What happens when r becomes 2r.

You get $$\frac{2}{3(2*r)^4} = \frac{2}{3*16*r^4}$$

How is this new expression different from the old one? The new one = Old one * (1/16)

Hence, s gets multiplied by 1/16 when r becomes twice.

Hi Karishma,

I have a small doubt here , I kept r to the left and s to the right and worked at it that way , below is the working :

Given $$3 r^4 = \frac {2}{s}$$
now r is doubled

$$(2r)^4 =\frac{2}{3s}$$
$$16r^4= \frac{2}{3s}$$

Now the above can be rearranged as $$3r^4= \frac{2}{16s}$$
on comparing this with the original it seemed that s is being multiplied by 16 when r is doubled , hence I chose E. What is the error in the logic here?
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- Stne
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Joined: 16 Oct 2010
Posts: 9446
Location: Pune, India
Re: In the equation 3r^{4}=(2/s), if r is doubled, then s must be  [#permalink]

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24 Feb 2018, 05:17
stne wrote:
VeritasPrepKarishma wrote:
irus wrote:
why 1/16 can someone explain pls

$$3*r^4 = \frac{2}{s}$$
$$s= \frac{2}{3*r^4}$$ (multiply by s and divide by 3r^4)

What happens when r becomes 2r.

You get $$\frac{2}{3(2*r)^4} = \frac{2}{3*16*r^4}$$

How is this new expression different from the old one? The new one = Old one * (1/16)

Hence, s gets multiplied by 1/16 when r becomes twice.

Hi Karishma,

I have a small doubt here , I kept r to the left and s to the right and worked at it that way , below is the working :

Given $$3 r^4 = \frac {2}{s}$$
now r is doubled

$$(2r)^4 =\frac{2}{3s}$$
$$16r^4= \frac{2}{3s}$$

Now the above can be rearranged as $$3r^4= \frac{2}{16s}$$
on comparing this with the original it seemed that s is being multiplied by 16 when r is doubled , hence I chose E. What is the error in the logic here?

s is in the denominator. You cannot say what it multiplies by.
To know "s is multiplied by ...", you need to separate out the s.
s = Old expression
New s = m * Old expression
You need the value of m.
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Karishma
Veritas Prep GMAT Instructor

Re: In the equation 3r^{4}=(2/s), if r is doubled, then s must be   [#permalink] 24 Feb 2018, 05:17
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