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# 5^x - 5^(x-3) = 124(5^y), what is y in terms of x? A x B x-6

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VP
Joined: 06 Feb 2007
Posts: 1022
5^x - 5^(x-3) = 124(5^y), what is y in terms of x? A x B x-6 [#permalink]

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23 Jul 2007, 16:20
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

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

Senior Manager
Joined: 28 Jun 2007
Posts: 454

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23 Jul 2007, 17:28
5^X - 5^X-3 = 124 5^Y

It is important to recognize 124 is 5^3 -1.

Once you realize that... you can reduce the L.H.S to

(5^3-1)* 5^-3 * 5^x = 5^3-1 * y

x-3 = y
Senior Manager
Joined: 28 Jun 2007
Posts: 454

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23 Jul 2007, 17:32
ioiio wrote:
5^X - 5^X-3 = 124 5^Y

It is important to recognize 124 is 5^3 -1.

Once you realize that... you can reduce the L.H.S to

(5^3-1)* 5^-3 * 5^x = 5^3-1 * y

x-3 = y

Hahahahah.... I've been toiling day and night for 2 years at work now. Yet to get a promotion. 2 days, a handful of problems, and I am already a manager in this forum

If only life can be this easy
GMAT Club Legend
Joined: 07 Jul 2004
Posts: 5043
Location: Singapore

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23 Jul 2007, 19:00
5^x - 5^x(1/125) = 124(5^y)
5^x(1-1/125) = 124(5^y)

5^x(124/125) = 124(5^y)
5^x = 125(5^y)
x = y+3
y = x-3
VP
Joined: 06 Feb 2007
Posts: 1022

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26 Jul 2007, 16:57
The OA is C. Thanks, guys!
26 Jul 2007, 16:57
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