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A student's average (arithmetic mean) test score on 4 tests

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A student's average (arithmetic mean) test score on 4 tests [#permalink] New post 26 Aug 2012, 03:07
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The Official Guide for GMAT® Review, 13th Edition - Quantitative Questions Project

A student's average (arithmetic mean) test score on 4 tests is 78. What must be the student's score on a 5th test for the student's average score on the 5 tests to be 80?

(A) 80
(B) 82
(C) 84
(D) 86
(E) 88

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Re: A student's average (arithmetic mean) test score on 4 tests [#permalink] New post 26 Aug 2012, 03:08
SOLUTION:

A student's average (arithmetic mean) test score on 4 tests is 78. What must be the student's score on a 5th test for the student's average score on the 5 tests to be 80?

(A) 80
(B) 82
(C) 84
(D) 86
(E) 88

Since the student's average test score on 4 tests is 78, then the sum of the first 4 test scores is 4*78=312. We need the sum of ALL 5 scores to be 5*80=400, so if x is the 5th test score, then 312+x=400 --> x=88.

Answer: E.
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Re: A student's average (arithmetic mean) test score on 4 tests [#permalink] New post 26 Aug 2012, 08:13
Let the score in the 5th test be x.
The sum of the first 4 tests = 78*4=312
Now it implies 312+x/5=80
Therefore x=88
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Re: A student's average (arithmetic mean) test score on 4 tests [#permalink] New post 27 Aug 2012, 11:15
(sum of x)/4 = 78
So (sum of x) = 312.
Now (sum of x + x5)/5=80
So (312+x5)=400 i.e. x5=88.
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Re: A student's average (arithmetic mean) test score on 4 tests [#permalink] New post 31 Aug 2012, 02:11
SOLUTION:

A student's average (arithmetic mean) test score on 4 tests is 78. What must be the student's score on a 5th test for the student's average score on the 5 tests to be 80?

(A) 80
(B) 82
(C) 84
(D) 86
(E) 88

Since the student's average test score on 4 tests is 78, then the sum of the first 4 test scores is 4*78=312. We need the sum of ALL 5 scores to be 5*80=400, so if x is the 5th test score, then 312+x=400 --> x=88.

Answer: E.
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A students average (arithmetic mean) test score on 4 tests [#permalink] New post 15 Dec 2012, 08:28
I follow Manhattan advice on this one and write the average formula as a first thing:

in order to reach an average of 80 over 5 test the students has to reach 400 points; he now has 78*4= 312.

400-312= 88.

Answer E
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Re: A students average (arithmetic mean) test score on 4 tests [#permalink] New post 16 Dec 2012, 04:17
Ans:
total score after 4 tests=312 and score needed after 5 tests= 5x80=400 the student scores the difference in the 5th test which is 88 , (E).
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Re: A student's average (arithmetic mean) test score on 4 tests [#permalink] New post 12 Apr 2013, 10:42
Examining the known average can simply things. We need to reach 80. We currently have 78, four times. 78 is -2 away from 80. Having this four times weights our average -8 off what is desires. To 'catch up' to the average in one test, we must make up the full 8 point deficit in one test. We need an 88. 88 - 8 = 80.
Re: A student's average (arithmetic mean) test score on 4 tests   [#permalink] 12 Apr 2013, 10:42
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