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M26-03

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M26-03  [#permalink]

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New post 16 Sep 2014, 00:24
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Question Stats:

72% (02:11) correct 28% (02:39) wrong based on 29 sessions

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If \(5^{10x}=4,900\) and \(2^{\sqrt{y}}=25\) what is the value of \(\frac{(5^{(x-1)})^5}{4^{-\sqrt{y}}}\)?

A. \(\frac{14}{5}\)
B. 5
C. \(\frac{28}{5}\)
D. 13
E. 14

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Re M26-03  [#permalink]

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New post 16 Sep 2014, 00:24
Official Solution:

If \(5^{10x}=4,900\) and \(2^{\sqrt{y}}=25\) what is the value of \(\frac{(5^{(x-1)})^5}{4^{-\sqrt{y}}}\)?

A. \(\frac{14}{5}\)
B. 5
C. \(\frac{28}{5}\)
D. 13
E. 14


First thing one should notice here is that \(x\) and \(y\) must be some irrational numbers (4,900 has other primes than 5 in its prime factorization and 25 doesn't have 2 as a prime at all), so we should manipulate with given expressions rather than to solve for \(x\) and \(y\).

\(5^{10x}=4,900\)

\((5^{5x})^2=70^2\)

\(5^{5x}=70\)

Now, \(\frac{(5^{(x-1)})^5}{4^{-\sqrt{y}}}=5^{(5x-5)}*4^{\sqrt{y}} = 5^{5x}*5^{-5}*(2^{\sqrt{y}})^2\)

Since \(5^{5x}=70\) then \(5^{5x}*5^{-5}*(2^{\sqrt{y}})^2=70*5^{-5}*25^2=70*5^{-5}*5^4=70*5^{-1}=\frac{70}{5}=14\)


Answer: E
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Re: M26-03  [#permalink]

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New post 02 Jul 2015, 07:53
Hi,

This solution path is hard to visualize for me, is there a shortcut or an aproximation method viable to solve this question faster? If negative, what is your best advice with this kind of weird expressions?

Thanks a lot

Regards

Luis Navarro
Looking for 700
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Re: M26-03  [#permalink]

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New post 02 Jul 2015, 08:09
2
luisnavarro wrote:
Hi,

This solution path is hard to visualize for me, is there a shortcut or an aproximation method viable to solve this question faster? If negative, what is your best advice with this kind of weird expressions?

Thanks a lot

Regards

Luis Navarro
Looking for 700


No shortcut there.

Study theory and practice questions from below links:
Theory on Exponents and Roots: math-number-theory-88376.html
Tips on Exponents and Roots: exponents-and-roots-on-the-gmat-tips-and-hints-174993.html

All DS Exponents questions to practice: search.php?search_id=tag&tag_id=39
All PS Exponents questions to practice: search.php?search_id=tag&tag_id=60

All DS roots problems to practice: search.php?search_id=tag&tag_id=49
All PS roots problems to practice: search.php?search_id=tag&tag_id=113

Tough and tricky DS exponents and roots questions with detailed solutions: tough-and-tricky-exponents-and-roots-questions-125967.html
Tough and tricky PS exponents and roots questions with detailed solutions: tough-and-tricky-exponents-and-roots-questions-125956.html

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Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: M26-03  [#permalink]

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New post 04 Jul 2015, 03:29
4√y=(2√y)2----- i dont get this part
Isnt (2√y)2== 2 (2√y)
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Re: M26-03  [#permalink]

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New post 05 Jul 2015, 07:35
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M26-03  [#permalink]

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New post Updated on: 27 Oct 2018, 02:28
I think this is a high-quality question and I agree with explanation.

Originally posted by rever08 on 26 Oct 2018, 12:07.
Last edited by rever08 on 27 Oct 2018, 02:28, edited 1 time in total.
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M26-03  [#permalink]

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New post 27 Oct 2018, 01:35
rever08 wrote:
I think this is a poor-quality question and I agree with explanation. The question leaves room for us to assume 4 = 2 * 2^√y or 4= (2√y)^2. But, then yes 28/25 isn't available as an option so we need to assume the latter.


Not sure how you got that but the question is fine and there is no ambiguity there whatsoever.

Could yuu please show how you get these values? Thank you.
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Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: M26-03  [#permalink]

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New post 27 Oct 2018, 02:30
Bunuel wrote:
rever08 wrote:
I think this is a poor-quality question and I agree with explanation. The question leaves room for us to assume 4 = 2 * 2^√y or 4= (2√y)^2. But, then yes 28/25 isn't available as an option so we need to assume the latter.


Not sure how you got that but the question is fine and there is no ambiguity there whatsoever.

Could yuu please show how you get these values? Thank you.



Ohh I made a brainless mistake :shh: . Thank you the question is clear now, have edited my post too.
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Re: M26-03 &nbs [#permalink] 27 Oct 2018, 02:30
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