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24 Jan 2016, 05:46
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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:
84% (01:26) correct
16%
(01:49)
wrong
based on 180
sessions
History
Date
Time
Result
Not Attempted Yet
If 15 students in a class average 80% on an English exam and 10 students average 90% on the same exam, what is the average in percent for all 25 students?
A. 83%
B. 83 1⁄2%
C. 84%
D. 85%
E. 86 2⁄3%
A. 83%
B. 83 1⁄2%
C. 84%
D. 85%
E. 86 2⁄3%
24 Jan 2016, 21:10
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Bunuel
The average percentage = (15 x 80% + 10 x 90%)/25
= (15 x 80% + 10x 80% + 10x10%) / 25 ......(as 90% = 10% + 80% )
=(25x80% + 10x10%)/25
= 80% + 10x10%/25
= 80% + 4%
= 84%
==> C
24 Jan 2016, 21:42
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Bunuel
Hi,
there could be two ways to these Qs..
1) intensive calculations :-
we multiply the averages with number, add the two and again find average..
15 students have 80% average..
lets take 80% as 80 marks,as we are dealing with only %..
so total marks=15*80=1200..
similarly 10 students 90 average marks= 10*90=900..
total marks= 1200+900=2100..
students=25, so average= 2100/25= 84%..
2) weighted average:- A better way with less calculations..
we have 15:10 students with 80:90 average %..
the final average will depend on the ratio of students and will be closer to the average with more students..
so the fraction/ratio of students with 90% marks= 10/(10+15)=2/5..
The final average will be (1-2/5)* the difference of the averages from 90%..
so 3/5*10=6 away from 90.. 90-6=84
C
