Question

In a class , the average marks of boys is 520 and that of girls is 420. If the average marks per student is 500,then the percentage 0f boys in the class is 1. 20% 2. 40% 3. 80% 4. 90%

Answer 1

Let the number of boys be x and that of girls be y.

total marks scored by boys will be= 520(x) = 520x
total marks scored by girls= 420(y) = 420y

Total number of the students = x + y
total marks scored by both boys and girls = 500(x+y) = 500x + 500y

Therefore sum total scored by girls and total scored by boys should be equal to that scored by the whole class:

520x + 420y = 500x + 500y

520x-500x=500y-420y
20x=80y
x=4y

That means number of boys = 4y
and that of girls = y

total of class = 4y + y= 5y
% of boys(4y):

4y (100%)          =    4/5 (100%)    =  4(20%)
5y

=  80%

Answer 2

Answer:

Let the number of boys be x and that of girls be y.

total marks scored by boys will be= 520(x) = 520x

total marks scored by girls= 420(y) = 420y

Total number of the students = x + y

total marks scored by both boys and girls = 500(x+y) = 500x + 500y

Therefore sum total scored by girls and total scored by boys should be equal to that scored by the whole class:

520x + 420y = 500x + 500y

520x-500x=500y-420y

20x=80y

x=4y

That means number of boys = 4y

and that of girls = y

total of class = 4y + y= 5y

% of boys(4y):

4y (100%)          =    4/5 (100%)    =  4(20%)

5y

=  80%

Step-by-step explanation: