Bunuel wrote:

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%

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

_________________

1) Absolute modulus : http://gmatclub.com/forum/absolute-modulus-a-better-understanding-210849.html#p1622372

2)Combination of similar and dissimilar things : http://gmatclub.com/forum/topic215915.html

3) effects of arithmetic operations : https://gmatclub.com/forum/effects-of-arithmetic-operations-on-fractions-269413.html

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