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Director
Joined: 16 May 2014
Posts: 592
Given Kudos: 0
GRE 1: Q165 V161
GRE Math Challenge #58-m^3 + 380 = 381m
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28 Feb 2015, 02:37
28 Feb 2015, 02:37
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58% (01:17) correct
41% (01:26) wrong based on 46 sessions
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If m is a negative integer and \(m^3 + 380 = 381m\) , then what is the value of m?
A. -21
B. -20
C. -19
D. -1
E. None of the above
A. -21
B. -20
C. -19
D. -1
E. None of the above
Retired Moderator
Joined: 10 Apr 2015
Posts: 6218
Given Kudos: 136
Re: GRE Math Challenge #58
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10 Apr 2015, 08:36
10 Apr 2015, 08:36
3
soumya1989 wrote:
If m is a negative integer and m³ + 380 = 381m, then what is the value of m?
A. -21
B. -20
C. -19
D. -1
E. None of the above
A. -21
B. -20
C. -19
D. -1
E. None of the above
Here's one approach:
Given: m³ + 380 = 381m
Rearrange to get m's on one side to get: m³ - 381m = -380
Factor to get: m(m² - 381) = -380
At this point, it's probably easiest to just PLUG IN the answer choices.
HOWEVER, I'm going to skip answer choice A since 21 is NOT a factor of 380, and we already know that -380 = m(m² - 381), which means m must be a factor of 380
Let's start with answer choice B (-20)
Plug -20 into the equation to get: (-20)[(-20)² - 381] = -380
Evaluate: (-20)[400 - 381] = -380
Then...: (-20)[19] = -380
Works!!
Answer:
Show: ::
B
Cheers,
Brent
