What is y in terms of x?

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mehravikas: Legendary Member; Posts: 1161; Joined: Mon May 12, 2008 2:52 am; Location: Sydney; Thanked: 23 times; Followed by:1 members

by mehravikas » Tue Sep 30, 2008 12:55 pm

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

Answer is: c

Source: Beat The GMAT — Problem Solving | Tue Sep 30, 2008 12:55 pm

anju: Master | Next Rank: 500 Posts; Posts: 258; Joined: Thu Mar 29, 2007 3:15 am; Thanked: 7 times; Followed by:1 members

by anju » Tue Sep 30, 2008 12:58 pm

5^x - 5^(x-3)
= 5^(x-3) (5^3 - 1) - take out common values
= 5^(x-3) (125-1)
=124*5^(x-3) = 124*5^y

so y = x-3

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