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If 5^x – 5^(x- 3) = 124*5^y, what is y in terms of x?

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If 5^x – 5^(x- 3) = 124*5^y, what is y in terms of x? [#permalink] New post 11 Feb 2011, 02:17
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If 5^x – 5^(x- 3) = 124*5^y, what is y in terms of x?

A. x
B. x - 6
C. x - 3
D. 2x + 3
E. 2x + 6
[Reveal] Spoiler: OA

Last edited by Bunuel on 20 Sep 2012, 13:15, edited 1 time in total.
Renamed the topic and edited the question.
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If 5^x – 5^(x- 3) = 124*5^y, what is y in terms of x? [#permalink] New post 13 Dec 2012, 02:25
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vaivish1723 wrote:
5^x – 5^(x- 3) = (124)(5^y), what is y in terms of x?
A. x
B. x - 6
C. x - 3
D. 2x + 3
E. 2x + 6

Oa is
[Reveal] Spoiler:
c


Finding one variable in terms of another is very simple. Assume a value for one and find the other. Then look for the option that works.

Say, x = 3 (so that x-3 becomes 0 and we are left with simple calculations)

5^3 - 5^{3- 3} = (124)(5^y)
1 = 5^y
y = 0

Now put x = 3 in the options and find out which option gives you y = 0. Only C does so answer is (C)
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Re: ps [#permalink] New post 11 Feb 2011, 04:13
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alltimeacheiver wrote:
If 5x – 5x - 3 = (124)(5y), what is y in terms of x?
A. x
B. x - 6
C. x - 3
D. 2x + 3
E. 2x + 6
Answer:


Question should read:

If 5^x – 5^(x- 3) = 124*5^y, what is y in terms of x?
A. x
B. x - 6
C. x - 3
D. 2x + 3
E. 2x + 6

5^x-5^{x-3}=124*5^y --> factor out 5^{x-3}: 5^{x-3}(5^3-1)=124*5^y --> 5^{x-3}=5^y --> bases are equal so we can equate the powers: y=x-3.

Answer: C.

alltimeacheiver format the questions correctly.
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Re: ps [#permalink] New post 18 Feb 2011, 11:17
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u can also plug in x=3. makes it very simple.
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If 5^x – 5^(x- 3) = 124*5^y, what is y in terms of x? [#permalink] New post 12 Dec 2012, 18:52
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{5^{x}}\frac{124}{125}=124(5^y)
5^{x}=5^{y+3}
x=y+3-->y=x-3

Answer: C
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If 5^x – 5^(x- 3) = 124*5^y, what is y in terms of x? [#permalink] New post 26 Feb 2014, 19:47
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5^x – 5^{(x- 3)} = 124*5^y

Re-write as

5^x – 5^{(x- 3)} = (125 - 1 ) * 5^y

5^x – 5^{(x- 3)} = 125* 5^y - 5^y

5^x – 5^{(x- 3)} = 5^{(y+3)} - 5^y

LHS = RHS, so equating the powers

y = x+3 ; Answer = C
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Re: ps [#permalink] New post 20 Sep 2012, 13:05
Bunuel wrote:
alltimeacheiver wrote:
If 5x – 5x - 3 = (124)(5y), what is y in terms of x?
A. x
B. x - 6
C. x - 3
D. 2x + 3
E. 2x + 6
Answer:


Question should read:

If 5^x – 5^(x- 3) = 124*5^y, what is y in terms of x?
A. x
B. x - 6
C. x - 3
D. 2x + 3
E. 2x + 6

5^x-5^{x-3}=124*5^y --> factor out 5^{x-3}: 5^{x-3}(5^3-1)=124*5^y --> 5^{x-3}=5^y --> bases are equal so we can equate the powers: y=x-3.

Answer: C.

alltimeacheiver format the questions correctly.


Hi Bunuel,
I', trying to understand your factorization, but I find some troubles when I have to factorize some value with exponent an expression like in the question (x-3 or x+4),
Could you explain me that or do you have a deck where I could dive into this argument?
Thank you in advance
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Re: ps [#permalink] New post 20 Sep 2012, 13:14
Expert's post
mario1987 wrote:
Bunuel wrote:
alltimeacheiver wrote:
If 5x – 5x - 3 = (124)(5y), what is y in terms of x?
A. x
B. x - 6
C. x - 3
D. 2x + 3
E. 2x + 6
Answer:


Question should read:

If 5^x – 5^(x- 3) = 124*5^y, what is y in terms of x?
A. x
B. x - 6
C. x - 3
D. 2x + 3
E. 2x + 6

5^x-5^{x-3}=124*5^y --> factor out 5^{x-3}: 5^{x-3}(5^3-1)=124*5^y --> 5^{x-3}=5^y --> bases are equal so we can equate the powers: y=x-3.

Answer: C.

alltimeacheiver format the questions correctly.


Hi Bunuel,
I', trying to understand your factorization, but I find some troubles when I have to factorize some value with exponent an expression like in the question (x-3 or x+4),
Could you explain me that or do you have a deck where I could dive into this argument?
Thank you in advance


5^{x-3}(5^3-1)=124*5^y --> 5^{x-3+3}-5^{x-3}=124*5^y --> 5^{x}-5^{x-3}=124*5^y.

Theory: math-number-theory-88376.html

Practice:
tough-and-tricky-exponents-and-roots-questions-125956.html
tough-and-tricky-exponents-and-roots-questions-125967.html

Hope it helps.
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

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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25 extra-hard Quant Tests

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Re: ps [#permalink] New post 24 Sep 2013, 08:07
144144 wrote:
u can also plug in x=3. makes it very simple.


Hi,

Can you please tell why you chose only 3 to substitute x with ?
Re: ps   [#permalink] 24 Sep 2013, 08:07
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