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A Simple but confusing DS [ 8a(1/4)b = ? ]

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Source: — Data Sufficiency |
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Re: A Simple but confusing DS [ 8a(1/4)b = ? ]

by stop@800 » Fri Oct 17, 2008 12:26 am
cold_cam wrote:8a(1/4)b = ?

(1) b = 1.5a
(2) a = 2

Lets see how do you guys approach this one :D
Qn is
8a(1/4)b
= 2ab

A
insuff

B
insuff


A+B
you can solve for A and B

so Ans is C
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thanks for quick reply Gmat Destroyer

by cold_cam » Fri Oct 17, 2008 12:42 am
Thanks for Quick reply.. but actually the question is



8^a (1/4)^b = ?

(1) b = 1.5a
(2) a = 2

its 8 to the power a and 1/4 to the power b.

I just copied and paste it so there was mistake Sorry
:(
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Re: thanks for quick reply Gmat Destroyer

by stop@800 » Fri Oct 17, 2008 12:55 am
cold_cam wrote:Thanks for Quick reply.. but actually the question is



8^a (1/4)^b = ?

(1) b = 1.5a
(2) a = 2

its 8 to the power a and 1/4 to the power b.

I just copied and paste it so there was mistake Sorry
:(
Now the Ans is A
Just simplify in terms of power of 2
you will get your ans
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hmmm

by cold_cam » Fri Oct 17, 2008 1:19 am
I agree, upon solving we end up with:

8^a.(1/4)^b

2^(3a).2^(-2b)

3a-2b = ?


But isn't is same as the S(1):

b = 1.5a
2b = 3a

and if we further see our original equation 3a-2b = ?
it tells us 3a = 2b which is what the statement (1) is. So is it solving the question.
I know how you will explain this as 3a-2b= 0 so its sufficient but that is exactly the same as question stem.
:roll:

why do we need s(1) to solve it, the question stem is solving it... I think I know why am confused but am still confused lol
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by stop@800 » Fri Oct 17, 2008 2:45 am
2a = 3b

hence 2a-3b = 0
and 2^0 = 1

this 2^0 = 1
is actually solving the question

Hope I understood your point correctly
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by 69aero69 » Fri Oct 17, 2008 3:54 am
1) B = 3/2 A

then,

8^A . (1/4)^(3/2 A) =
=8^A . [(1/64)^A]^(1/2)=
=8^A . [(1/8)^2A]^(1/2)=
=8^A . (1/8)^A=
=1

therefore, Suf

2) we know nothing about B, therefore Insuf

Hence, the answer is A
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by 69aero69 » Fri Oct 17, 2008 3:55 am
we it appears 8), it should be "8 )"
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by msvmuthu » Fri Oct 17, 2008 9:06 am
You can solve as given in the image[/url]
Attachments
fact.JPG
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Re: A Simple but confusing DS [ 8a(1/4)b = ? ]

by yezz » Sun Oct 19, 2008 4:03 pm
8^a(1/4)^b = ?

(1) b = 1.5a
(2) a = 2

2^3A * 2^-2B = 4^3A-2B

FROM ONE

2B = 3A THUS 3A-2B = 0 THUS THE EXPRESSION = 1

FROM 2

INSUFF

A
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by cbrombe1 » Tue Oct 21, 2008 10:59 am
Thanks everyone for your explanations. My biggest problem is with the arithmetic (1/4 ^ 1.5a). So if I take (1^1.5a)(4^-1.5a), how do I get to the next step?
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by cramya » Tue Oct 21, 2008 7:44 pm
Like msvmuthu explained u still would have to that (3^3a-3a = 2 6 0 =1)

If u do it like u r saying u will get

2^0 * 1 ^ 1.5a

1 raised to any exponent is always 1 so answer is 1

Anyone?
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by cramya » Tue Oct 21, 2008 7:44 pm
Like msvmuthu explained u still would have to that (2^3a-3a = 2 ^ 0 =1)

If u do it like u r saying u will get

2^0 * 1 ^ 1.5a

1 raised to any exponent is always 1 so answer is 1

Anyone?
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by sudhir3127 » Tue Oct 21, 2008 11:14 pm
cramya wrote:Like msvmuthu explained u still would have to that (2^3a-3a = 2 ^ 0 =1)

If u do it like u r saying u will get

2^0 * 1 ^ 1.5a

1 raised to any exponent is always 1 so answer is 1

Anyone?
its A.. exactly the way muthu explained...
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by java_ka_jalwa » Wed Oct 22, 2008 3:55 pm
I dunno if its easier , but 1.5 = 3/2 and (1/4)^3/2 is cube of square root of 1/4 that is 1/8 so we have 8^a * ( 1/ 8 )^a which is 1^a = 1
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