Bunuel wrote:

If 20% of a class averages 80% on a test, 50% of the class averages 60% on the test, and the remainder of the class averages 40% on the test, what is the overall class average?

A. 80%

B. 74%

C. 58%

D. 56%

E. 50%

Apart from the calculations method we can get down to two choices by merely looking at the info..

20%-80%marks..

50%-60% marks..

30%-40%marks..1)Here once we see the max concentration is 60% and the other two sets are at equidistant from 60% with slight bias towards 40%.

so our answer has to be slightly less than 60..

A and B are straightway out as they are >60.. E too is far off ..

finally we are left with only C and D.. now we are left with only 2 to answer..

2)

Lets do it by weighted average..first take weighted average for outer values..

20%-80%marks..

30%-40%marks..the difference 80-40=40 is divided into five equal parts (due to ratio 20:30)..

so each part is 8.

since 40% has more weight we will add lesser of 2 parts: 3 parts to it..40 + 2*8=56..

now we have two set of people..

50%-56%marks..

50%-60% marks..equal numbers for 60 and 56, so ans (60+56)/2=58

C

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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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