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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?
A. x
B. x - 6
C. x - 3
D. 2x + 3
E. 2x + 6
just assume 1 value of x like I assumed the value of x to be 4 . 5^4-5^1=620 now try to assume the value of y in such a way that the rhs also comes out to be 620. so y should be 1 in order to get 620 on the rhs. so answer is y-3
commented Apr 24, 2018 by Beginner (3 points)

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This may seem to be a very easy question, but make sure you dont do it in your head for two reasons
1) Dont Tax your brain, with additional computation to be geared up for english section
2) Avoid Silly Mistakes
Let us factor out the LHS
=> 5^X - 5^ ( x - 3)
=> 5^x - ( ( 5^x ) / ( 5^3) )
=> 5^x ( 1 - 1/ 125 )    ----- Taking 5^x as common and 5^3 = 125
=> 5^x ( 124 / 125)
Comparing from RHS . 124 gets cancelled out. The exponent of 5 has be equal on both sides. so
y = x - 3
Hence the asnwer is C
answered Apr 2, 2015 by Guru (5,628 points)
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