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C
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!
Difficulty:
15%
(low)
Question Stats:
83% (01:39) correct
17%
(02:00)
wrong
based on 274
sessions
History
Date
Time
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Not Attempted Yet
If the average (arithmetic mean) of a,b,c is 5, the average (arithmetic mean) of c,d,e is 8, and the average (arithmetic mean) of a,b,c,d,e is 7, what is the value of c ?
A. 2
B. 3
C. 4
D. 5
E. 6
A. 2
B. 3
C. 4
D. 5
E. 6
02 Mar 2017, 11:54
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amit10th
Dear amit10th,
I'm happy to respond.
Often, the secret to thinking about average questions is to compute the sum. We can't add or subtract averages, but we can add and subtract sums. See:
GMAT Averages and Sums Formulas
average = (sum)/(# of terms)
so--
sum = (average)*(# of terms)
a + b + c = 5*3 = 15
c + d + e = 8*3 = 24
Add those two equations
a + b + 2c + d + e = 15 + 24 = 39
Meanwhile, we know the average of a, b, c, d, e is 7, so
a + b + c + d + e = 7*5 = 35
Subtract this from the line above:
(a + b + 2c + d + e) - (a + b + c + d + e) = 39 - 35
c = 4
OA = (C)
Does all this make sense?
Mike
General Discussion
02 Mar 2017, 12:22
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amit10th
\(a + b + c = 15\)
\(c + d + e = 24\)
\(a + b + c + d + e = 35\)
Now, \(2a + 2b + 3c + 2d + 2e = 74\)
Again, \(2a + 2b + 2c = 30\)
So, \(( 2a + 2b + 2c ) + ( c + d + e ) + ( d + e )= 74\)
Or, \(30 + 24 + ( d + e ) = 74\)
Or, \(d + e = 74 - 54\)
Or, \(d + e = 20\)
We know, \(c + d + e = 24\) and \(d + e = 20\)
So, \(c = 4\)
Thus, answer will be (C) 4
