Question

The mean weigt of 60 students is 40 kg. If the mean weight of boys is 50 kg, while that of girls is 30 kg. Find the numbers of boys and girls.

Answer 1

let the number of boys be 'x'
and girls in the school be 'y'.
therefore x + y = 60
since mean = sum of observations by no of observation

50*x + 30*y / x + y = 40
 50x + 30y = 40*60
50x + 30y = 2400
5x + 3y = 2400/10 = 240

so we have 5x + 3y = 240 and x + y = 60
multiplying second equation by 3 , we get ,
5x + 3y =240
3x + 3y = 180
by subtracting , y terms will be cancelled , and 
2x = 60 
x = 30

substituting x in 2nd equation , we get,

30 + y = 60
y =  60 - 30 = 30
hence x = 30 and y = 30