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If (4^x)(8^y)(16^7x) = (8^4y)(32^x)(64^y) what is value x/y?

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If (4^x)(8^y)(16^7x) = (8^4y)(32^x)(64^y) what is value x/y? [#permalink] New post 05 Jan 2014, 06:39
00:00
A
B
C
D
E

Difficulty:

  25% (medium)

Question Stats:

82% (02:38) correct 18% (03:12) wrong based on 34 sessions
If (4^x)(8^y)(16^7x) = (8^4y)(32^x)(64^y) what is the value of x/y?

A. 3/5
B. 2/3
C. 3/4
D. 1
E. 4/3

Hi,
I want to know whether there is quicker solution for this question, please.

[With standard math]
As (b^c)^d = b^cd, and (b^c)*(b^d) = b^(c+d)
(2^2x)(2^3y)((2^4)^7x) = ((2^3)4y)(2^5x)(2^6y) → (2^2x)(2^3y)(2^28x) = (2^12y)(2^5x)(2^6y)
(2x + 3y + 28x) = (12y + 5x + 6y) → (30x + 3y) = (18y + 5x)
(25x = 15y) → (5x = 3y)
(x/y) = (3/5)
[Reveal] Spoiler: OA

Last edited by Bunuel on 05 Jan 2014, 06:44, edited 1 time in total.
Edited the question.
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Re: If (4^x)(8^y)(16^7x) = (8^4y)(32^x)(64^y) what is value x/y? [#permalink] New post 05 Jan 2014, 06:48
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goodyear2013 wrote:
If (4^x)(8^y)(16^7x) = (8^4y)(32^x)(64^y) what is the value of x/y?

A. 3/5
B. 2/3
C. 3/4
D. 1
E. 4/3

Hi,
I want to know whether there is quicker solution for this question, please.

[With standard math]
As (b^c)^d = b^cd, and (b^c)*(b^d) = b^(c+d)
(2^2x)(2^3y)((2^4)^7x) = ((2^3)4y)(2^5x)(2^6y) → (2^2x)(2^3y)(2^28x) = (2^12y)(2^5x)(2^6y)
(2x + 3y + 28x) = (12y + 5x + 6y) → (30x + 3y) = (18y + 5x)
(25x = 15y) → (5x = 3y)
(x/y) = (3/5)


(4^x)(8^y)(16^{7x}) = (8^{4y})(32^x)(64^y);

(2^{2x})(2^{3y})(2^{28x}) = (2^{12y})(2^{5x})(2^{6y});

2^{2x+3y+28x} = 2^{12y+5x+6y};

2x+3y+28x=12y+5x+6y;

25x=15y;

\frac{x}{y}=\frac{3}{5}.

Answer: A.

Theory on Exponents: math-number-theory-88376.html

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

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Re: If (4^x)(8^y)(16^7x) = (8^4y)(32^x)(64^y) what is value x/y? [#permalink] New post 25 Feb 2014, 22:27
LHS >> 4, 8, 16 all are powers of 2 re-written as 2^2, 2^3 & 2^4 respectively

RHS >> 8,32, 64 all are powers of 2, re-written as 2^3, 2^5 & 2^6 respectively

After making bases same, powers can be equated:

2x + 3y + 28x = 12y + 5x + 6y

25x = 15y

x/y = 3/5 = Answer = A
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Re: If (4^x)(8^y)(16^7x) = (8^4y)(32^x)(64^y) what is value x/y?   [#permalink] 25 Feb 2014, 22:27
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